Virtually all wheel departures are due a failure of the wheel hub. The wheel nut, safety bolts, wheel bearings and spacers are all found on the axle post flight. This is primarily due a failure during the NDT inspection proccess, or lack thereof, when the wheel assembly is removed and sent for tire replacement. I can not speak for other airlines but L-UAL had a state of the art tire and brake shop at the MOC in SFO for decades. The wheel came off the aircraft, sent to SFO and was overhauled, including NDT inspection of the wheel halves by UAL mechanics. That shop was closed and the work farmed out to the lowest bidder in the early 2000's during bankruptcy. Infer from this what you will.