What would cause intermediate shaft failure.Could it be hard driving or could it be high miles.

 

Or C) Faulty design. Seems to me that it's usually a high cycle failure (happens over a long period).
I don't think that it happens often with low mileage engines. This is typical of a classic metal fatigue situation. Time will cause it, hard driving can possibly accelerate it.

 

Thanks for the answer.Helpful information