Its too early for me to work out the physics behind it but belts and things like that often tend to fail at low revs becasue the loadings on them are higher. My dad had an old Rover SD1 2.6 and that seiezed its cam shaft at tickover (twice actually but thats another story).
Years ago when i used to build Mini engines with hot cam shafts they needed to be run in at highish revs otherwise it was possible to knock the lobes off the cam.