Why would serpentine belt and pulley come off ?

To me it sounds like the pump or pulley is bad they are a press fit. I would run it into a local shop and see what they say.

Damaged p/s pulley or pulley incorrectly installed. These power steering pulleys do not just “come loose” .. In fact that can be difficult to remove using the proper tools .. Also can be equally difficult to re-install using the *proper* tools!