Same thing happened to me many years ago In my HX Sandman. Castle nut completely stripped off to the point of being smooth where the thread should have been. Had driven this van to just about everywhere, Melbourne to Brissy numerous times, Hotham every weekend during winter and it decided to "Let Go" reversing out of car park at South Land shopping centre HA!. A good mechanic friend of mine was at a loss at what caused it to happen. The van had had a pretty tough life with a few front end bingles from previous owners. Found that the sub chassis had a Crack that had gone unnoticed and I put it down to that. When thinking of all the places I'd been that it could've happened really freaked my out a bit.