What makes a rear hub assembly go bad?

Time and mileage, abuse (not greasing it when you should). Also if you live in the northern states where it sees a lot of salt and snow will make it wear out faster. Mostly thought its just mileage that makes them die. Anything over about 100k miles don't be surprised, but if you're lucky and take good care of your car like i do I've got 180k miles and they're still fine.