Originally Posted by Andrew R Stewart

Precession is the term for the forces that rotate pedal axles within the crank arm threads. The threading directions used are those that make the axle self tightening. By using a reversed from the convention threading direction (as in the OP's LH axle in RH arm) that pedal axle will tend to self loosen. Once a pedal axle is slightly loose then the next type of forces kick in. The pedal will be wiggle this way and that way within the arm and the hard steel axle bearing the full weight of the rider will auger out the arm's hole. IME it's his two step process that causes the real damage, removal/flowing away of the arm's material. Andy