Often happens when the bearings are in poor shape. With good bearings, the torque you apply to the pedals gets reversed and tries to tighten the threads. (That's why left side pedals are left-hand thread.) When those bearings aren't working, your torque is now trying to unscrew the pedal. (On a fix gear, if that pedal actually freezes up, you better either have no foot retention or hope that mechanic didn't give the wrench that last tug because you will be unscrewing the pedal with the bones in your foot.)