Zab loses balance because of the foot placement, yes. But Floyd doesn't. The feet getting tangled have very little effect on his balance, because his momentum/body is going forward when the feet hit - which wouldn't have made him go falling back in the direction he did. Only if he was backing up in the opposition direction of Zab, would that tangling of the feet be the cause of him losing balance like that.
It was the punch that did it. Legit knockdown.