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Crankshafts rarely break. I would say that a lot of the time is because of the journals wearing out or something getting in the engine (like a valve breaking, water leaking into the engine and locking it up, etc.). Timing belts can cause this problem too. If it was not installed properly then it could have caused a valve to break and fall off into the cylinder and, in turn cause the crank to break (You can usually tell if this is what caused it by taking the engine apart and analyzing it). Also, sometimes the harmonic balancer can cause this if it is faulty.
I think the percentages of these problems are fairly similar. I think they all can cause the crank to break about the same amount of time.

From my experience these are some of the most common causes of crankshaft failure.