My analysis - and others on the internet like Smartfootball.com's Chris Brown - is that LSU seems to have been in a zero blitz. That is, a blitz with no safety deep and man coverage across the board. It's a pretty "en vogue" call these days because of tweaks to the philosophy to it. It isn't a "no ifs, ands, or buts" 7-man blitz. Each blitzer who ends up on an eligible receiver reads the blocker: if he immediately releases into a route, they peel off the blitz and trail the receiver. This makes the 7-man calls more sound against your traditional hot route throws to a receiver at the end of the line or in the backfield and ultimately makes the blitz more effective because it forces the QB to throw downfield to longer developing routes and likely take the sack.
The key is that Alabama's staff anticipated a blitz with some kind of peel principle on the eligibles in the box. They didn't call an immediate off-tackle screen, it was a delay screen of sorts where the receiver gives a solid engage on the blitzer then released at just the right moment where the defender got too deep into his rush to think anything but "hit QB." McCarron sold it well by having excellent timing looking off - reinforcing to the free blitzer that he just didn't get duped by Yeldon - and turning towards Yeldon right when he was ready to catch the screen pass.
This is an example of getting a team in just the defense needed to execute the play. The success of the last few plays kind of made LSU unlikely not to execute the pressure they were showing since their defense was reeling and Alabama had just the play up their sleeves to bust it.