I think we all wondered that initially, given that the aircraft was the last to crash and clearly did so some way short of itīs intended target.
However, if the aircraft had been shot down then the debris field would be very much bigger than it is since the aircraft would almost certainly have broken up at altitude.
The more plausible scenario is that the pax tried to regain control of the aircraft. We know that pax on this plane were in touch with relatives (on the instructions of the hijackers) on cellphones an airphones. It is also known that these relatives would have been aware of the attacks on the Twin Towers and possibly the Pentagon since UAL93 was hijacked later and was still enroute to Washington.
If, as is likely, the pax learned from these relatives that the hijackers intended to crash the aircraft killing not only them but many more people on the ground, then the blades carried by the hijackers and even their threat to detonate a bomb lost their potency as a means to keep the pax timid. If you know with all certaintly that you are going to be killed unless you risk attacking the hijackers then you will attack. Mark Bingham, the guy who called his mother and sister, was heard to say to a fellow passenger "...itīs true" before ringing off."
Maybe control of the aircraft was lost in the struggle, maybe the hijackers had some kind of bomb (explaining the second crash site six miles from the other) but it seems clear that the pax on UAL93 fought for their lives and unfortunately lost.
[ 13 September 2001: Message edited by: B747wideboy ]