Royce,
I believe it is due just to vibration normal use. The ones I have seen crack are about in line with the tip of the propeller which can have sound waves coming off of it. Not sure if that has anything to do with it but its one theory.
All of the planes currently on the production line have that part made of a different material. Can't remember if they changed the thickness and the alloy or just the alloy. It's a softer material now that is not as brittle so it can take the vibration better. This change happened a while back but I don't know exactly what s/n.