Happened to mine, too, fortunately not far from home, washer and circlip lost. I limped back, trying to hold the pedal onto its shaft with my foot, and fitted approximately correct washer and circlip until I could buy the correct parts. I suspect it's really the consequence of wear on the washer and pedal shaft, and old age of the circlip. In the end, I decided to replace the shaft, washer and circlip with genuine parts. No further problem .... but I do keep an eye on it all, and grease the shaft, regularly now.