another thing i just learned about spd's, if that's what you have....they can disengage from pulling straight up with too much pressure. you don't necessarily have to twist your foot. i've clipped out on accident a couple of times while throwing down a skid very suddenly or sprinting from a trackstand. i thought it was just a matter of adjusting float, but no.

so if they're spd's, just be wary of them. they're certainly not foolproof.