HOLY COW!
It would take a hellava lot of torque to do that!!
Do you reckon that the axle was damaged - and that became the stress riser? (Where the splined shaft inserts into the socket) The pic looks like it wasn't pure brittle fracture, and that there's a fair bit of plastic deformation.
I like the swirley shear pattern on the shaft too! - best set of pics in the forum i reckon.
I've heard of cases at work, where rubber seals on big mining mill shafts have worn a really shallow groove - and the shafts have failed as a result.
Or maybe you just have a killer donk unda da hood! LOL
Best of luck getting her back on the road!