There are a couple ways the threads are stripped. Pedals are right/left specific - the left pedal is reverse threaded. Try forcing the left pedal in the right side and the right pedal in the left side and you'll damage, if not strip, the threads.
Also, if the pedal is not tightened down it can possibly unthread. When not all threads are engaged, downward force on the pedal can rip it from the cran arm, stripping the threads - usually on the crank arm.