Originally Posted by bmulls
It's because you have a heavy upper body. I have the same problem. When your back comes off the floor there's a moment where you are basically see-sawing on your butt. If you are carrying any appreciable amount of muscle your torso is going to weigh more than your legs and you'll fall back.
That has to have something to do with it besides my somewhat weak abs.
Because I've seen fat out-of-shape women with thunder thighs doing unassisted situps, and I doubt it's because they have strong abs. Their lower body weight helps them stay grounded.