Glass is a funny substance like that. Not too long ago my dad had a drinking glass sitting on the kitchen counter (empty), we heard a crack and looked over and it had broken apart just sitting there. It hadn't even been filled with anything, he had just removed it from the cabinet and put it on the counter.
This is one of the reasons I don't trust glass carboys, even if you never bump or bang them they will just one second decide to break. Better bottles FTW!