That sucks, but yeah thats usually one of 3 things:

a) tension
b) not enough float (i.e. you unclip because of too much heel angle)
c) not clipped in the first place... I've had this problem with a single shimano pedal... it felt like it was clipped it but it wasn't and would eventually release. finally realized that if I didn't hear a click, it wasn't good.

Otherwise I've sprinted hard enough to bring the front wheel off the ground w/o unclipping, and MTBers race on them all the time.. so...