Brady addressed the play in his weekly D&C spot. He said there were discussions on the play before the game. It was purely a "situational play" if they saw the cover-2 look in the right spot. They knew that the safety would have trouble getting over in that snow.
The plan was to send Gronk up the seam and Branch on the out. Brady would do a little pump-fake to hold the safety and if he wasn't fading out towards Branch, he'd take the shot knowing the safety could never get back in those conditions.
Worked to perfection. Gronk went up the seam (clean off the line), the safety stayed inside when Brady did a little pump, and that left Branch open on the sideline, and no way for the safety to reverse field on the release of the ball. Of course, they thought they'd be just beating man-to-man, but Tillman got lost on it and Branch was WIDE OPEN.
Situational ball at it's finest. Obviously a design play that required the QB to recognize the situation and make the adjustment, not just to Branch, but to Gronk (to run the seam) and the Oline (to set protection). They would have taken the knee,but I'm sure Brady has the freedom to run one of those situational plays whenever he sees the look that warrants that designed play. It was something they saw pre-game on tape and discussed (with the coaches) before the game. It just came up at a point where the sideline coaches were content to go into the locker room.
Probably happens in every game 1 or 2. just not with such spectacular results.