I have noticed this when I first rack to fermenter from bk, but never after fermentation, unless you are fermenting in a bucket and have pressed on the lid to get a smell, then the bucket would try to pull air back in
Simple... active fermentation produces a lot of heat. It can raise your wort 5 degrees above ambient. Once it ends, it begins to cool down to ambient. Cold gas takes up less space than warm gas, thus the shrinkage/vaccuum.
Must be part of the problems I've noticed lately with my S-type airlocks. Topping them off twice a day, and they still were turning up empty. I've switched to the other type, and they stay full without a problem.