Anyone care to guess what could have caused this axle to break?
Loading on the freehub. Ie, somebody cased a landing/cumulative damage. Same thing that causes 7 speed+freewheels to break.
stick wedged between frame and cassette?
I don't see a machined radius where it changes diameter. That would make a stress riser, and cracks to develop over time.
The wear on the tapered piece is likely from the able flexing/rubbing the sleeve on the freehub. The bearing looks like it is recessed.
If a larger bearing doesn't sit on the large part of that taper, I'd say it's a garbage design.