Improperly torqued U-Bolts could cause this. There was a spring shop in NY many years ago that was repairing buses and failed to properly torque the U-Bolts causing this to happen to many buses.
Another fault, which happened to me in another life, is the vendors u-bolt has a tendency to be too brittle, and the bolt breaks at the bend. As a note, I don't use that vendor any longer!