If the defender had continued to initiate contact to keep Fouss away from the basket, it would not have been an offensive foul. But, on the last hit from Fouss, he was just standing there. Fouss knocked him over. He took a chance that Fouss would make another strong move to the basket. The gamble paid off for him.

If Fouss had decided to make a different move, the defender would have been stuck out of position and Fouss would have scored easily. It's just the game post-defenders have to play with an offensive player who is strong enough to back them down.