My guess would be crevice corrosion of the stainless steel. In a closed, corrosive environment such as bolt threads or where the bolt shank passes through the hole, the stainless steel can't develop its protective layer and it eventually pits or corrodes internally almost like tiny worms in wood. 316 SS helps, and so does a liberal dose of Never Seize which helps to keep water out of the crevice, but even with that, routine inspection and replacement of SS fasteners is a good thing.