OK. Maybe I need to rewatch it. I thought the runner was already past. If not that makes sense.
Yeah, catch is right... that is the standard operating procedure for those types of run plays.
However, typically what you see when a small DB sees a 325 pound guy running at them is for the defender to sort of hunker down because he knows he can't hold his ground against a guy like Smith. When they hunker down, the OT usually just falls over the DB, causing a bit of a pile up, like Catch mentioned.
What you don't see usually, is the defender running full speed and diving at the OT's knees as if he were the running back. It was designed to make the pile up but to also hit the knees of the OL much harder than is necessary.
You don't see that often. I'm not even sure there is a rule against it but if it happened often there would be a lot more knee injuries by pulling OL and the player(s) doing it would get a dirty reputation fast.