Sounds to me like the bolts sheared. Like Jim said, possibly from over torquing. Use the torque specs, and a little thread locker.
A pet peeve of mine: Hardware Gorillas; people who think tighter is always better. Ironicly I have "gorilla proof" hex wrenches.

Daggerdoggie: probably from accumulated fatigue on the metal

DBAX: That is the coolest graemlin thing ever.