Yep, stress fatigue. The only way this would happen would be a take off in opposite directions and the boat moving in a sideways manner or a take off all to one side with a 90 degree angle to the Bow, the boat would be tugging from one side.
You should find a good selection of U bolts from hardware internet sites like here
or Here at the Bottom of the page
This should get you started. Just check the dimentions from the online drawings in the websites to identify which one is suitable.
If this might happen again, it might be an idea to look for a different soloution than using a U bolt?