Simple -- the receiver pushed off the defender (hand in the chest) to make space to catch the ball. That's cheating, and was rightfully flagged. It should be called more often in the NFL, but this was 1) blatant and 2) right in front of the ref.
All I saw was Jackson's (I've got to defend my fellow Gator, after all :)) arms go under the Steeler defender, I never saw the hands touch the defender's chest in a way that allowed Jackson to push off, or stopped the momemtum of the defender. And the defender doing the exact same thing to Jackson, in fact, you could have made a case for defensive pass interference on that play.