Mark Barron as dime backer
In dime packages, Mark Barron plays in the box as a sort of linebacker with Ahmad Black and Dashon Goldson often playing deep. Barron's skills vs both the run and pass allow him to be effective in this role.
The Patriots need 3 yards
The Buccaneers are playing what looks like a three-deep, three-under defense with Dashon Goldson picking up the most vertical threat to the three-receiver side. The Patriots are trying to stretch the coverage to create an open man. Note that I may be wrong on what the Bucs are doing in coverage, as you'll see in the next screenshot.
Barron on Bolden
Mark Barron is the key here -- the ball is going to Brandon Bolden with Barron in coverage. Also note how every player is matched up with a player -- this signals that either they're playing a matchup zone, where the receivers pick up and stick to whoever comes into their zone, or that it's a form of man coverage with some odd drops and David playing an underneath zone in the middle. This is why it's hard to discern exactly what's happening in coverage. The fact that most of the receivers weren't picked up until they were well beyond the first down marker, and the fact that David and Barron passed off their initial players lead me to believe it's a matchup zone.
Barron makes the play
Despite being quite a distance away, Barron uses his burst and length to break up the sure first down pass.
Pressure gets to Brady
While it doesn't look like much, if Brady holds onto this ball for half a second longer, Bowers or McCoy would get to Brady. Those two have been the only consistent pass rushers for the Bucs, with McCoy playing much better than Bowers.