"I'm just a caveman, thawed out by your scientest", but here is my observations based on the photos.

It appears to be a fatigue fracture that initiated at multiple locations (Stage I-fracture initiation) shown my the red arrows, although these are only examples that can be verified by closer examination. The ratchet marks(white arrows) form as a result of multiple fracture planes and lends support to more than one fracture origin. The flat portion of the fracture (dark bracket) and, to some extent, the woody appearance of the fracture (dashed bracket) are a result of progressive crack advancement (Stage II- crack propogation). Final separation, or overload region, occurred in the cleaner-looking dull gray area that is shown as Stage III. Some evidence of beachmarks (blue arrows) indicate progressive crack growth before final separation (again, points to fatigue).

It is unclear if any material deficiency or pre-existing defects exists without looking at it closer.

I my past experience while driving in a 69 Dart with the torsion bars raising the front end quite a bit we experienced the same thing. That fracture had a classic torsional break of a round bar in a brittle material(spiraling fracture). However, that fracture had a tiny thumbnail near the origina (Stage II), and a rock ding at the fracture origin.

Bones break in a similar manner when torsionally loaded (see below), and you can recreate a torsional fracture using chalk or a crayon (also shown below) by twisting it in opposite directions just for fun.