Axles (Not axels BTW) break. It could be from a single big impact, but usually is the result of metal fatigue that happens over time.
If you look at a schematic of a rear hub you'll find that the axle is supported at either end, and subject to loads inboard of the ends (depends on hub design). The loads are both the axle weight and from the chain pulling the hub forward. That flexes the axle, and eventually it'll fail at a thread - usually the first one inboard of the right cone or bearing.
It's been eight years, so hopefully for not a lot of dough you can go another eight years, if something else doesn't fail first.
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