This is what kinda did it for me. But yeah...maybe a engine fell off...

Originally posted by Zaphod58
Wild side to side maneuvering puts strain on the engine mounts they're not designed to take. Engine mounts handle vertical stress very well (up to over 5Gs on a 747 I heard about) but they don't handle lateral stress well at all. Dutch rolls were enough to take off two engines on a KC-135, and cause serious damage to the other two engine mounts. We know flight 93 was doing some wild maneuvering to try to throw the passengers off their feet, and it went over to an inverted position at some point before crashing.