Well, a couple things here. First, I think he was waiting (as I was) for the challenge flag, since the play before that was a touchdown by him, that should have been overturned. Even Tomlin said after the game they think it would have been overturned but did not get word down in time to challenge it.
So, I think for sure that was the reason, and then it looked like he was down. But, a lot of players there were kinda stopped waiting for the whistle. It was a clear touchdown, with 2 feet down then the defender swiped down to knock the ball out. But, clearly it was a catch and control, and thus a dead ball in the end zone per the rule.