pully came off on 1992 Audi 100

pulley stripped of water pump off engine, what was the cause and are the enigmas easy to find at junk yards

Asked by for the 1992 Audi 100
pulley is part of the water pump, it came off because it's
19 years old. a brand new water pump is less than $100, so
why would you get one at a junk yard?