OK, here's my guess. They had it out for maintenance and had the arc chutes out. They have open and closed gap settings to check so they use a feeler gage. Somehow they get the arc chutes back in with metal stuck between phases, They shouldn't have even had access to the 3/4" bolt or square drive to crank if the breaker was closed. So unless a failed shutter of some kind caused the fault, I'll bet they left hardware in the breaker.
Joe
