The same reason a person croaks after getting too old. Once you reach a certain age, you either begin to have health problems, or you just flat out die of old age. Computer hardware is the same way. Sometimes, it'll just croak on you. Others, it'll die a slow, painful, agonizing death. Now, i'm not saying that's whats happening with your GPU, but what i am saying is that just because something works for a period of time, doesn't mean it'll keep working. Otherwise things would last for ever, including a persons life :P.