They didn't quit -reality set in! In the first half Tenessee was not getting dominated by the Fla defense, the receivers were dropping balls that should have been caught. Had the Tn drives been extended by better execution the score could easily have been 21-20 at half or even 14-21.
The Fla defense had not been tested until the Tn game. Their pastry teams are really weak and Ky is a basket case. The Fla players must have believed their hype just as their fans did. The back half of their defense is going to look like an overcooked breakfast before the end of this season.
Second half the Tn defense stepped it up a notch which the Fla offense could not match (9 yards of offense in the 3rd qtr). Meanwhile, the Tn offense began to click (28 points in five minutes). The Fla defense got manhandled in that stretch (I think their DBs were trying to find the Tn receivers 10 minutes after the game was over).
Reality can be harsh but Fla defense is no where near "nation's number one".