The factory u bolts were designed to distort the threads when it was torqued the first time. That made a positive locking effect. Some will distort better than others, but they are still a one time use bolt by design. Will a used u bolt re-torque and be good a second and third time....its happened. Some of the ones that had to be torched off may have done a better job doing what they were designed to do.
Just for safety, I do not re-use U bolts for street duty. I have several used ones on my cars for the sake of mock up builds or yard cars that only get driven around the yard, however I mark used U-bolts with a splash of chebby orange paint and they get replaced when the car gets approved for regular to heavy street use.
Not saying U bolts were the cause but......why take a chance with bolts when new ones are available and designed to work like the factory ones.