I'm not familiar with ARB bumpers, but I assume the bolts came with it for the install? You're probably right about the thing just being stressed over time, but still seems odd to break the bolt. Go to someplace like tractor supply and replace it with some grade 8's. Then drive around town looking for the fool who ran into you and test the stress of the bolt out on their vehicle.

Glad everyone was ok. Here's hoping the cops can track down the culprit.