Sometimes a shaft can just fail. Metal gets lots of cracks in it when under stress, but if they don't make it to the surface of the metal they seal themselves (people don't realize that perfectly clean metal will bond with itself). If the crack makes it to the surface, then the impurities prevent it from re-sealing and the part fails. This is why shot-peening is used to strengthen metal parts--it makes it much harder for the crack to reach open air, or water, or an oil bath, so it re-seals more often.

The stuff you learn in 34 years of motorcycle riding!

The other suggestions were to save you from unnecessary dis-assembly by ruling out other causes.