Spokes break from incidents, poor manufacturing/QC, and from fatigue. I suspect the latter did this spoke in. Rental bikes (like autos) lead a hard life. If the wheel had been treated poorly in the past, then trued/retrued a few times, and used over and over by riders who didn't baby the wheel I can see very easily why a spoke might break with out any other influence.

Wheels with fewer spokes, narrower rims, lots of dish (rear wheels with lots of cogs), poor initial build and follow up tensioning/truing, and big riders who pedal squares all make matters worse. Andy.