It seems that forge (version 7.7.0.587) is causing snow to behave differently than vanilla minecraft. When pushed with a piston normally the snow would disappear, but with forge installed it pops up as a snowball. It also lets snow golems make snow on top of hoppers, which doesn't seem to happen (at least as fast) in unmodded minecraft. Here's a video showing the bug, where I got excited and made a machine for it thinking it was a new feature:
There are other mods in the video, but I tested with just forge and it still happens.