Random tragedy...any idea why regular car wheels don't go ballistic...SHEAR FORCES on big vehicles? lack of maintenance! crappy parts being used?............ 
If you notice it is always the trailer wheels that go flying off, not any part of the tractor tandems.
Two reasons:
1. Tractors are invariably better maintained than trailers.
2. But by far the more significant reason is that when turning, sometimes while going forward, but mostly and especially when backing up and turning at the same time the back tandems often twist in the same place as the truck pivot's around e.g. to back up into a loading dock. That places tremendous shear forces on those back wheels as they rotate while in place. When new they can withstand that force but as they get older and with exposure to salt and corrosion they can snap off easily, anywhere, including on the highway.
To get around this problem, they will have to change the law and make it mandatory to replace the back trailer tandems of every trailer say every 3-5 years. It will be expensive but the only way I see to make this problem go away. It is physically impossible to evaluate how much stress a wheel assembly will take before it snaps in any kind of daily inspection. I doubt whether even a qualified mechanic will be able to definitely certify a wheel assembly as road worthy based on a visual inspection.
