Perhaps there was some metallic "spare part" that fell into the socket or made its way between the terminal and screw shell when the bulb was changed out?
Reminds me of some stupid pranks some kids in my college dorm used to do to the wall sconces in the stairwells. Those had light bulb sockets facing up, no enclosure around the bulb. Someone would steal the bulb to replace a blown bulb in their desk lamp, and someone else would drop a penny into the empty but live sconce socket. And cause sparks and trip the breaker... idiots.
