Great Football Outsiders article says it better than I could:
The Titans’ game-tying drive involved the game’s oddest play. On fourth-and-10, Kiwanuka had Young in his grasp. Young made a pump-fake, and Kiwanuka, fearful of a roughing-the-passer penalty, released Young, who proceeded to scramble for a first-down.
This odd looking play has led to some condemnation for Kiwanuka, but this gaffe was certainly understandable. The NFL has seen a number of questionable roughing penalties, although not an overall increase. Had Young thrown the ball and Kiwanuka tackled him, the rookie would have been everyone’s goat for picking up a personal foul. A Tennessee drive had already been extended on a personal foul when Young was stopped short of a first down.
When he released Young, the quarterback still had over 15 yards to run for a first down, but Young evaded all tacklers. Will Demps had a perfect opportunity to make a game-winning tackle but missed. Demps’ failure is more cause for reproach than Kiwanuka’s.
Young’s feint raises a difficult problem for referees and the Competition Committee. Already, it seems that some quarterbacks are testing the rules by delaying their slides to the last minute or tiptoeing down the sideline, almost trying to induce the penalty. Protecting the quarterback is an important objective, but when players stop making legitimate plays for fear of a flag, the situation needs to be reexamined.
Believe it or not, that’s only a fraction of this detailed article.