I've had pedals fall off from a combination of the bearings failing and not so tight installation. Lesson - check bearings. I don't carry pedal wrenches unless I"m doing a very long unsupported tour (and probably not in this lifetime).
The fact that the pedal unscrewed is rather reassuring for this fix gear rider. (It happened onmy road bike so it was a complete non-issue other than getting myself home.) If a fix gear pedal's bearings froze and the rider's foot was securely clipped in and the pedal didn't unscrew, that could be rather bad. (Let's spin this rider clockwise by his foot!)