A photo of the errant tire, with police car for scale.
The brakes act directly on the rim so that the wheel cannot impart a load to the retaining nut. The retaining nut has two keeper bolts to keep it from turning and there is a hubcap that looks to me like it would prevent the two bolts from backing out of their holes. The hubcap protects the connection to the pressure monitoring system and has a band clamp to hold it in place.
I lean towards a fatigue crack in the axle where the retaining nut sits.The axle turns with the tire and is subject to cyclic loads. In contrast, car axles are fixed in rotation and the bearings and wheels rotate. The crack would be at the bottom of a thread or the inside corner where the threads stop - a place where plenty of brake dust would settle and cover the crack. I notice in the following video the axle is not scrubbed clean or specifically inspected.
It looks like the hubcap is missing, maybe from the impact load.
Interesting info about tire install: