It looks like a multi-mode failure. The origin is near the top of the axle shaft in the third picture. Faint lines point to the failure origin. These faint lines are indicative of fatigue mode and the smoother fracture surface on the 1/3 outer radius of the axle shaft seems to be all fatigue.
The rough or pitted-looking area in the middle is caused by brittle fracture. The small spot of rust in the middle may be caused by flash rusting once it's exposed to the air/moisture. The rest didn't rust because, I suspect, gear oil covered the area almost immediately after failure.
The sequence is is initiated at the origin; there was probably a surface imperfection (a nick, a gouge, or any other crack-like feature caused during a manufacturing/assembly process). This creates a stress concentration area which caused fatigue to set in. After numerous cycling of acceleration and deceleration, the crack propagated within area of the smoother surface ring. Once the area was large enough, the last failure mode was brittle fracture which happened all at once. The remaining area was simply overstressed and aggravated by the sharp crack tips surrounding it.
This is a design of a semi-floating axle. In a full-floating axle, such axle shaft failure will not cause loss of control as the hub is retained by the axle housing/tube.