Failures as a result of an error in the manufacturing processes isn't unknown, despite the many checks put into place to prevent them.
It's quite possible the hub fractured as a result of a manufacturing flaw that wasn't picked up in the manufacturing checks.
The report says the wheel hub separated from the MLG (one would presume the actual axle), so that generally means the entire wheel and hub becomes completely detached.
More importantly, did the crew start singing, "You picked a fine time to leave me, loose wheel!" .....
