Why should space whisky taste different to Earth whisky?
From the press release from Suntory, the Japanese company which sent 6 vials of their whisky into space, which details their study:


  Tokyo, Japan, 30 July -- Suntory Global Innovation Center is about to embark upon space experiments on the “development of mellowness in alcoholic beverage through the use of a microgravity environment.” This research will be conducted in the International Space Station’s Japanese Experiment Module (nicknamed “Kibo”), with the cooperation of the Japan Aerospace Exploration Agency (JAXA).
  H-II Transfer Vehicle No. 5, commonly known as “Kounotori5” or HTV5, is scheduled to be launched from JAXA’s Tanegashima Space Center on August 16 (Sunday) carrying alcohol beverages produced by Suntory to the Japanese Experiment Module aboard the International Space Station, where experiments on the “development of mellowness” will be conducted for a period of about one year in Group 1 and for two or more years (undecided) in Group 2.
  
  With the exception of some items like beer, alcoholic beverages are widely known to develop a mellow flavor when aged for a long time. Although researchers have taken a variety of scientific approaches to elucidating the underlying mechanism, we still do not have a full picture of how this occurs.
  
  Our company has hypothesized that “the formation of high-dimensional molecular structure consisting of water, ethanol, and other ingredients in alcoholic beverages contributes to the development of mellowness,” and we have been conducting collaborative researches on this topic with research groups of Professor Shigenao Maruyama of the Institute of Fluid Science, Tohoku University and Professor Mitsuhiro Shibayama of the Institute for Solid State Physics, the University of Tokyo, the Japan Synchrotron Radiation Research Institute and Suntory Foundation for Life Sciences. The results of these collaborative researches have suggested the probability that mellowness develops by promoted formation of the high-dimensional molecular structure in the alcoholic beverage in environments where liquid convection is suppressed. 
  
  On the basis of these results, the space experiments will be conducted to verify the effect of the convection-free state created by a microgravity environment to the mellowing of alcoholic beverage.
  
  The Suntory Group aims to use these experiments to help find a scientific explanation for the “mechanism that makes alcohol mellow.”


It appears that the working hypothesis is that the lack of gravity in space precludes liquid convection and thus alters the characteristics of liquids aged in that environment ${\it vs.}$ that on Earth. As you indicate, it's allegedly one way that promotes the formation of compounds which are otherwise not formed when gravity and convection within the ageing liquid are present.

From the same press release, details about the study:


  Title of study
          Elucidating the Mechanism Mellowing Alcoholic Beverage
  
  Experimental period
          Group 1: August 16, 2015 (Sunday) (planned) to September 2016 (planned)
  Group 2: August 16, 2015 (Sunday) (planned) to undecided date two or more years later
  
  Content of experiment
          One set of samples consisting of various alcoholic beverages will be stored in a convection-free state in Japanese Experiment Module “Kibo” on the International Space Station, and another set of the identical samples will be stored in Japan for the same period of time. The following methods will be then used to analyze and compare the two sets of samples.
  
  
  Measurement of substance diffusion coefficient with the use of a phase shifting interferometer$^{1}$ in cooperation with the Institute of Fluid Science, Tohoku University.
  Detection of high-dimensional structure by small angle X-ray scattering$^{2}$ using SPring-8, in cooperation with the Japan Synchrotron Radiation Research Institute and the Institute for Solid State Physics, the University of Tokyo.
  Measurement of substance diffusion with the use of the NMR method$^{3}$ in cooperation with Suntory Foundation for Life Sciences.
  
  
  [1] Phase shifting interferometer: Uses the properties of light (visible light) waves to visualize temperature and concentration distributions, etc. that cannot be seen by the eyes, making it possible to detect slight changes in the ingredients of alcoholic beverages and determine how they are distributed.
  
  [2] Small angle X-ray scattering: Can be used to obtain nanoscale (1 millionth of a meter) structure by irradiating a substance with X-rays and observing the scattered X-rays in an angular region of a few degrees or less.
  
  [3] NMR method: Uses the phenomenon of nuclear magnetic resonance (NMR) to extract information from the organic compounds that are the object of measurement. Can be used to obtain information on the molecular structure of a compound and its dynamic properties.
  
  Samples
          Five types of distilled spirits differing aging periods and 40% ethanol: Total of six samples


Suntory is not alone. Ardbeg scotch made a similar trip into space, and you can read about the experiment and results here. They used GC, HPLC, GC-MS, and organoleptic assessment to analyze their samples.

As stated in their summary:


  The key aims of the experiment were as follows:-
  
  (i) To determine if conditions of micro-gravity would have an impact on the range of naturally occurring terpene compounds and on the extraction
  of key flavour-active compounds in oak wood by Scotch malt distillate.
  
  (ii) To assess the impact of maturation in conditions of micro-gravity on the flavour profile of oak matured distillate.
  
  (iii) To investigate the presence of novel compounds and flavours in model maturation systems for Scotch malt distillate, as a result of conditions
  of micro-gravity.
  
  The key findings of the above three aims would then be considered in conjunction with actual maturation conditions on Earth to investigate
  techniques for developing novel flavours in Ardbeg Islay Single Malt Scotch Whisky.


In the Results and Discussion section (I do encourage interested parties to jump to the link, and there are several tables and too much information to post here), they note:


  GC analysis of major volatile congeners demonstrated no significant differences between the International Space Station samples and the control
  samples on Earth. Since this group of compounds comprising primarily alcohols, aldehydes, ketones and esters, are produced during fermentation
  and concentrated up, or in some cases, catalysed during distillation, their presence and quantities should have been identical in each set of samples
  to start off with. The conditions of micro-gravity on the ISS have not impacted on their levels, as demonstrated by the similar levels in the Earth
  based samples (Table 1.)
  
  A more detailed investigation of these compounds, along with phenolic compounds, was carried out on our behalf by external whisky experts and
  scientists, using GC-MS. In the resulting chromatograms, the areas of 75 separate peaks were integrated, most of which were identified as alcohols,
  esters, acetals and phenols. Again, no major differences were found between the ISS and Earth samples. From an Ardbeg perspective, this was
  particularly interesting as the conditions on the ISS have not impacted on the absolute quantities of the various phenolic compounds. These are
  the compounds which contribute the powerful, smoky, medicinal flavours to Ardbeg spirit.
  
  The most significant variable between the ISS and Earth samples was discovered when the results of the HPLC analysis of key maturation related
  congeners (wood extractives) were considered. The absolute concentration of these compounds was far higher than would normally be expected
  in standard, barrel-matured spirit, almost certainly as a result of the much higher surface area of woody material that the spirit was exposed to in
  the MixStix. However, when the two samples, ISS and Earth, were compared to each other, the lignin breakdown products - which are the major
  flavour active derivatives from oak wood - were present in higher concentrations in the control samples than in the space samples. This indicates
  that conditions of micro-gravity have inhibited their extraction from oak. On the other hand, ellagic acid and gallic acid, which are tannin breakdown
  products, both appeared to be higher in the ISS samples. The levels of these compounds are detailed in Table 2.
  
  By drilling down further into these results, and subjecting them to some statistical analysis, the relative abundance of the main, easily extractable
  lignin breakdown products (vanillin, coniferaldehyde and sinapaldehyde) expressed as a percentage of the total wood extractives, is broadly
  similar in each sample. However, other extractives, such as the syringyl compounds, are very different in their total abundance, with much lower
  percentages in the ISS samples. This result means that while the overall extraction of wood-derived compounds is negatively impacted by micro-gravity,
  the impact is not equal; the compounds can be subdivided into ‘readily extractable’ and ‘less readily extractable’.
  
  While it is clear that micro-gravity is inhibiting the release of wood extractives, not all compounds are affected equally, as would be expected
  in normal conditions. This has resulted in an unusual ratio between the readily extractable compounds and the less readily extractable ones in
  the ISS samples. This offers the intriguing possibility of using the ratios of such compounds as a potential marker to identify ‘unusual’ maturation
  characteristics, and thus a means of identifying for example, spurious age claims in whiskies. This possibility may be further investigated by the Scotch
  Whisky Research Institute.
  
  For studies such as this, where we are examining the impact of differing conditions on either model systems, or on the full scale in actual barrels,
  the key question is always ‘What does this actually mean for the flavour?’, so the organoleptic analysis was of great interest to us. The results from
  the key volatile congeners, wood extractives and phenols analysis did not lead us to believe that from an organoleptic perspective, there would
  be large differences between the ISS and control samples. However, when we finally nosed and tasted the samples in the sensory laboratory,
  the results were unexpected.
  
  Triangle tests gave a high number of correct responses, with almost all participants picking out a difference between the ISS and control samples.
  When we examined the actual aroma and flavour profiles, the differences between the two were as remarkable as they were unexpected; all tasters
  described the Earth control samples as being of the general Ardbeg whisky style, while the ISS samples displayed a quite different set of flavours.
  These have been summarised in the detailed descriptions below.


The above-mentioned MixStix are described in the write-up:


  MixStix™ technology, which provides a convenient platform for small scale sample experimentation, is a trade-marked piece of technology used
  by NanoRacks. Essentially, they are small glass vials which can hold a range of materials; in our case these materials were 6ml of Ardbeg new spirit
  distillate, along with a small quantity of charred oak wood shavings (from a once used American oak Bourbon barrel), c. 1mm x 1mm x 6cm in
  size. The two materials are separated from each other by a glass dividing partition, which is broken by bending the Teflon outside covering of the
  MixStix™ at time of experiment activation, thus allowing the two separate components to mix together.