As Brady gets set to throw, the CB is still running with Moss, and the safety actually bites in as though he's going toward Welker. They tried to make it seem as though the safety was helping the nickel back on Welker. Actually, I think Butler was so far out of position not only because of Moss's speed but because their plan was to make it seem as though he was doubling Welker. You can clearly see that Butler is moving toward Welker BEFORE Brady sets to throw the ball (at the same time the CB is running stride for stride with Welker).
I'm convinced by what you all have written about Brady's pre-read, but in the video it's clear that Brady makes these decisions because he knows what they are going to do, rather than reacting to what they actually do. Neither the safety Butler moves toward Moss nor the CB peels away from him prior to Brady's throw.