Bottom bracket bearings aren't just spinning in one direction and then the other (like a hub), there's also a lot of multi directional force being put upon them by the rider. Think about the immense pressure put on the bottom bracket of a fixed gear bike on any given day (except evilcryalotmore's bike). Take a pen or similar object and spin it back and forth in your fingers. Easy, right? Now bend the pen while spinning it.

I think it has most to do with the stock GXP BB not being very good to begin with. Also, the way the spindle goes through the external bearing cups doesn't seem that strong to me - especially when compared to a traditional square taper setup. A heavy-hitting friend of mine uses GXP on his trials bike and he has developed play in his crank that can't be tightened (and is getting worse).

I'm much more in favor of keeping the bearings inside the BB shell.