The usual way that pedal threads become stripped is from riding the bike with loose pedals. If the pedal spindle moves around loosely inside the crank arm the threads become deformed. The crank arm alloy is usually softer than the pedal spindle so that those become stripped first. Another way to strip the threads is to "cross-thread" the spindle into the crank arm. Be sure that the spindle is perpendicular to the crank arm while starting the spindle into the crank.
edit: Had not seem post #3 when typing my post.