Oxidation of acetic acid in wine?
The oxidation/reduction reactions of 

ethanol &lt;-&gt; ethanal &lt;-&gt; acetic acid (ethanoic acid)


are at equilibrium. Oxidation (presence of $\ce{O2}$ or other oxidizing agent) and reduction (addition of $\ce{H}$ to the $\ce{C-O}$ carbon) are in constant flux. The system can be pushed to the right if wine is exposed to air for long periods of time, as you found, but at a certain point it will reach equilibrium, in which case the excess of acetic acid will reduce (marginally) back into ethanal and/or ethanol.

A better explanation may be that since the B.P. of ethanol (78 dec. C) is significantly lower than acetic acid (118 deg. C), the vapor pressure of ethanol is greater and it has a higher affinity to convert to a gaseous state. This also would shift equilibrium to the left to produce a lower post-equilibrium concentration of acetic acid.

Acetic acid does have the possibility of decarboxylating, but this is usually in excess heat. In this case, a $\ce{CO2}$ molecule breaks off of the acetic acid leaving $\ce{CH4}$, which would also be generated as a gas.

Finally, it is possible that you're mistaking "loss of acetic acid" for an actual increase in pH, irrelevant to the presence of whole acetic acid molecules. It could be that other materials in the wine "soak" protons to increase pH and leave ethanoate counter ions ($\ce{CH3COO-}$). You should clarify how you're actually quantifying the concentration.

I wouldn't expect the "decrease" of acetic acid to be significant, compared to the amount that was produced in the first place. I would also expect your system to re-equilibrate after a while longer. It would be interesting to see what happens next.