>>eloisa+(OP)
If you watch more closely you'll see that this "almost good cop" shoved Gugino very hard, and is probably the reason he fell. This officer was holding his baton in both hands, and pushed both arms forward quickly just as the officer that "pulled him by the vest" braced him from behind for a very forceful shove. This happened at the exact same time the foreground officer straight-armed Gugino.
That combination would probably knock most people off their feet.