What I Thought Would Happen
What I thought would happen was for the pigs to just move around on flat snow, doing nothing strange or weird.
What Happened Instead
As the pigs moved around the flat snow, it kept jumping as it thought it was a whole block. This does not work with any other blocks such as buttons or slabs that I could find. This problem also works with other mobs, such as Chickens and Cows.
How To Reproduce The Problem
Place a couple of flat snow in an area, spawn some pigs and they will sometimes jump when walking up to them.