Not just weight on the 3ph, I would guess a lot of weight PLUS a hefty jolt.
Someone hit a hole going a mite too fast with a heavy implement on the back is my guess.
That or a hoe, but you didn't say if it ever had a hoe on it.
I'm betting someone had the valve assembly off and did not tighten the bolts back down. Bolts backed out, stripped out of the trans casing, valve flopped around under load, and was history.
