Why is a pig-skin like substance floating on top of my vinegar?

That looks like a symbiotic culture of bacteria and yeast -- a SCOBY.

As to why a SCOBY developed in one vessel and not the others, only guesses can be made. I found a blog post that may offer an explanation:

So a SCOBY is typically a SCOBY for all starting vinegar and kombucha fermentations when slow processes like vat or barrel fermentation are used. Both terms can work at this point. However, for vinegar around 1% acidity the yeast die off and at higher acidities the lactic acid bacteria die off as well leaving only acetic acid bacteria to feed on alcohol. At this point it is only a mother of vinegar containing one type of organism. Kombucha acidity usually ends between 0.5-1% while vinegar is 4% minimum and usually 5%. So calling a fermenting vinegar mother a SCOBY isn’t strictly accurate.

It may be the case that the yeast grew faster than the lactic acid bacteria and the solution never reached >1% acidity in that vessel. I would be interested to know if the pH of that vessel is higher than the others.