Are there any theories why such an imbalance in chirality of molluscs?
Why so many molluscs exhibit sinistral winding?

The estimates of the number of molluscs vary quite greatly between 50,000 and 200,000 species. Of those molluscs species, about 70'000 are Gasteropoda. Gasteropa is most diverse Mollusca phylum.

The winding you describe is present in all Gasteropoda and is often called the torsion. So the answer to why there are so many molluscs that make a torsion is simply, because of phylogenetic independence. The torsion evolved only once in the gastropods. The answer to the more specific question, why is the torsion right handed rather than left handed in all gastropods is 1) phylogenetic independence again. The torsion evolved only once and was therefore either sinistral or anti-sinistral. There is no need for an explanation of why they are all sinistral because the observations are not independent.

Why has the torsion evolved at first place?

One may ask but why did the torsion evolved at first place?. I think the reasons are still to be discovered. The following is a summary of what I read on wikipedia (torsion#evolution)

"Why torsion is bad"

As a result of this torsion, the anus is found next to the mouth which is an obvious hygiene issue and therefore seems rather deleterious. Moreover, there are a whole bunch of issues about organs spinning around and entwining. Also, ventilation seems to be reduced by the torsion which is pretty deleterious.

"Why torsion is good"

However, because there's no "hole" left in the posterior position, the torsion may help preventing sediments. Some have suggested that the torsion may allow to move sensory organs closer to the head. The most likely explanation is that the torsion might have evolved as a defense mechanisms against predation as torsion allow an organism to hide its head behind its shell. Finally, citing from wikipedia:


  The evolution of an asymmetrical conispiral shell allowed gastropods to grow larger but resulted in an unbalanced shell. Torsion allows repositioning of the shell, bringing the centre of gravity back to the middle of the gastropod’s body, and thus helps prevent the animal or the shell from falling over.


Note also that


  Whatever original advantage resulted in the initial evolutionary success of torsion, subsequent adaptations linked to torsion have provided modern gastropods with further advantages.


Why sinistral rather than anti-sinistral

To repeat myself, we only have one single observation of torsion (as it evolved only once) and this observation is either sinistral or anti-sinistral. It sounds therefore quite likely that stochastic processes have driven the evolution of sinistral (rather than anti-sinistral) torsion. In other words, the first mutation allowing for some degree of torsion was probably causing a sinistral torsion and this is it.

But there might eventually be a more functional reason for why torsion evolved to be sinistral. The reasons would then be related to the already existing asymmetry of organs. For example, anti-sinistral torsion may yield to more entwining between the gut and the respiratory system, or to not squeeze too much the one lung, who'd be smaller than the other one due to the presence of circulatory organs. I don't have enough knowledge in the anatomy of the molluscs ancestors to have a good intuition of whether I'd expect sinistral or anti-sinistral torsion to be more beneficial.