Hmmm... how many ways can you crash... here's some general categories I've seen, I'm sure you can add to the list!

1) Dumb-thumbing it into the ground - pilot looses perspective and pulls up instead of down, etc.
2) Unexpected aerodynamics - pilot causes an unintended stall or snap, and can't recover in time.
3) Unexpected obstacles - pilot's depth perception tells him he's cleared the trees, but no [:@] .
4) Random equipment failure - mechanical or electronics failures in flight causing loss of control.
5) Stressed equipment failure - pilot pushes airframe or servos harder than intended causing failure.
6) Getting shot down - two pilots switch on a TX of the same channel, and one of them is flying.