Just a guess, which I have not seen anywhere: if the pax know they are goners anyway (learning of the previous suicide crashes via cellphones), I would be willing to believe that they stormed the cockpit and --unable to move or disable the hijackers-- simply screwed things up so the plane would go in without taking other people with it.
"If we are going to die, and we are, at least let's not take anyone else with us. We can't take over and land safely. Let's let the pax pilot do what he can to put us nose down in an unpopulated area."
As with others here, I post this meaning no disrespect to anyone involved. I have thought often of this, as an old friend was on United 175. What would I do if I were the first one through the door and there was no way we could take over? I hope I'd be able to leave a voice recorder that says, "We can't overpower them. We're taking it in. God Bless America!"