With what has been indicated by the audio snippets that were heard, we also have to remember this - the people on the first flights may have been told that they were going to land the plane, and that if they cooperated they weren't going to get hurt. Obviously, that was bullshit.
The last flight went down, and word has it that it was the passengers revolting. They probably figured out that they weren't going to live, and said to hell with it, bumrushing the terrorists. That's what seems plausible to me.