not that this means a whole hell of a lot, but i've heard two other accounts of this happening to people. when i was toying with the idea of getting a clipless setup, a friend of mine told me that he had a pedal blast off of the spindle. i then mentioned it to someone else who said that they had a similar problem but noticed that the nut had come loose while lubing their chain before a ride. i am not too familiar with the pedals from personal experience, but it seems like it might be a good idea to inspect them periodically and make sure that the retaining nut or whatnot is secure.