It broke because it was flexed beyond it's elastic limit.
This is obvious, of course, like stating a ship sank because it could no longer float.
Frames are case hardened to provide a durable surface, while allowing a certain amount of flex.
The 'proof test' in this case proved where the frame was brittle.
The root cause of the failure is as simple as that. The question is was this caused by fatigue and did it present a danger going forward.
My personal opinion is no, and no.
A microscopic analysis of the failure and adjacent areas may conclude otherwise.
Until somebody does that, we're just guessing.
