non-drive side? can't speak to as why the threads on the crank bolt are bad, but the fact it spontaneously backed up is highly suggestive of a bad crankarm. the bolt was probably not torqued tightly enough to begin with, and the crankarm worked its way loose, ratcheting the bolt out along the way. once this happens, the taper on the crankarm is almost always ruined, resulting in a repeat event if the bolt/crankarm are reinstalled.

if it appears the crankarm and bolt will take, you should re-torque the bolt after a week or so of riding. otherwise you need a new arm.