Broken shaft on flywheel, JD D105.

[rsm]

What would cause this to happen

 

[bertsmobile1]

Metallurgically speaking , the tensile force encounters by the cast iron boss was higher than the very low tensile strength of the grey cast iron.

As to why, well it is your mower, what were you doing at the time it broke ?

 

[cpurvis]

What would cause this to happen

A very abrupt stop of the engine. There may be more damage than what you see in the photo.