[James]

I don't know what type of axle you have. But:

If you have a semi-floating rear axle, then you have a clip inside the differential that had somehow came loose. Opening up the rear cover and looking inside the carrier where the spider gears is at, you may find the reason for the failure.

If you have a full floating axle, then the nut came loose inside the hub allowing the axle to decouple.