Man, that's one way to ruin your whole day .. 😞 .. the operator was obviously travelling at speed, for the tractor to flip?
The pivot shaft appears to have been cracked across more than half its diameter, for a long period of time. Regular careful inspections of a tractor, looking for leaks, cracks, loose fasteners, items out of alignment, or unusual wear patterns, will often pick up things like this. However, the area where the crack is, is not the easiest place to see or find cracks.
This kind of failure is rare, but not unknown. While you are in charge of powered equipment, there's always the possibility of some mechanical failure event that can come out of left field, and catch you unawares. I think any of us that have operated equipment for lengthy periods of time, have had them.
Rapid response to emergency situations calls for quick reflexes, a cool head, training, and maturity. Another good reason for ensuring you don't let your 6 or 7 yr old operate a tractor unsupervised. I trust the operator was O.K.