[travelin texans]

I would take lots of photos as well as a picture of the label on the axle & contact the axle manufacturer, to me this is a manufacturing defect regardless of rust or an other outside causes, the wheel/drum should not break off clean like that.
I'd give the scenario you had a pucker factor of a full 10!!
Our son recently bought a new 12-15' enclosed trailer & while towing on the interstate had your same experience with both drivers side wheels pass him, after some serious "Oh sh##" moments he got stopped & found the centers of both wheels had broken out leaving the centers & lug nuts behind. There was an older man behind him that stopped & told my son he had to shake his hand cause he thought they were goners a couple times during the ordeal. His was also a 10 on the pucker scale!