That was a super single that still had the hub in it. The most likely chain of events was that the wheel seal failed, the bearing seized, the bearing clonked around until there was nothing left and then the whole wheel and hub came off together.

When I worked on trucks I saw a few of these, but most drivers caught them before the hub came all the way off. The friction would make them get so hot that the tires would explode and they usually noticed that. Repairing these was awesome. There was a mobile machinist that would come out, cut off the end of the spindle, weld on a new threaded end, grind down and weld to build up the gnarly bit of the spindle, and then regrind the whole thing concentric. Watching him work is what got me interested in machining.