Yeah, I think passenger game-theory is the number one reason, and that was already in place on September 12th, 2001.

Passengers are now aware that hijackers might actually just buying time until they can trigger a murder-suicide attempt, and many will believe it is likely enough that they need to fight for their own lives. The old assumptions that "almost everyone gets out alive by passively cooperating" no longer hold.

This happened on the 11th. See United 93.