Originally Posted by gyldenlove
The rule in question is the completed catch rule:
The section to pay attention to is C, the ruling is that he gained possession of the ball and maintained possession for long enough to advance it prior to breaking the plane of the end zone, this means soon as the tip of the ball cross the plane the play is over and anything that happens subsequently is irrelevant.
As for the common misconception that the ground can not cause a fumble that is an utter fabrication caused by TV commentators with insufficient knowledge of the rules of the game. If a player has possession of the ball and he goes to the ground in the field of play and contact between the ground and the players hand or the ground and the ball prior to the player being down by contact causes the ball to come lose it is a fumble.
It all comes down to wether or not the referee thought the receiver made the catch and then crossed the goal line. I'm going to have to watch the replay again, but it was a really tough call to make.