I just upgraded to Yosemite and my current best interpretation of what's happening is that there's a new behavior that "helpfully" moves the mouse (and a window if you're moving one) closer to an edge of the screen when moving quickly enough.
I really dislike this, in great part because it causes very strange behavior when I'm playing a game.
Is this a setting that can be changed?
With additional testing, it looks like the mouse doesn't snap to the edge, but has tremendous acceleration in some cases (quick changes in direction?) which it didn't have before the upgrade to Yosemite.