As for a cause, the only idea we have is that the aircraft at some point might have been out of rig and was flown with a constant control pressure in one direction. The constant pressure combined with the natural vibrations of the airframe may have caused the bolt hole to wear on one side.
Or in perfect rig, and always flown with one heavy person. N99387 goes dead straight
with two people. It drifts ever-so-slowly left with just me.
Greg
