The ruling of referee Walt Coleman: while Brady was trying to tuck the ball, his arm was moving forward, thus the play was ruled an incomplete pass. This was based on Rule 3, Section 21, Article 2, Note 2 of the NFL rule book: "...any intentional forward movement of [the passer's] arm starts a forward pass, even if the player loses possession of the ball as he is attempting to tuck it back toward his body."
Explained Coleman: "When I got over to the replay monitor and looked it was obvious that his arm was coming forward, he was trying to tuck the ball and they just knocked it out of his hand. His hand was coming forward, which makes it an incomplete pass."
Asked to explain what it would have taken for it to be ruled a fumble, Coleman said, "He would have had to have brought it all the way in and got it all the way underneath his arm in order for it (to be a fumble)."
"I don't understand how that play is looked at," Jon Gruden said. "But I thought it was a fumble."
Charles Woodson's reaction: "It's bull----. I thought it was bull---- call. It never should have been overturned."