I would say that the bolts were not properly torqued by whomever pulled them last. I've never had any bolt back out on me in more years than I care to admit of driving, and I've put most of those bolts back in after working on whatever I happened to be driving at the moment.

 

 

I agree that the issue is probably improper torque. But use loctite. Gm uses it from the factory, and it is highly recommended you do the same

 

As these parts tend to corrode make sure the threads are clean on the bolt and also chase the threaded holes with a wire brush (gun brushes do the job). Also make sure the threads are not damaged and don't have too much clearance. Brakes are vital parts.

If it is corroded and you put corroded bolts back together, sometimes the bolts reach the prescribed torque long before they are clamping the caliper to the support at the proper specs.

New bolts would be a good way to go.