Besides overloads, shorts,age and heat from fellow breakers, What else will cause a breaker to trip?
Edward
Heat from a loose connection to Bus or Load.
Vibration
I've seen bad contacts on an a/c contactor heat up breakers to the point of tripping.
At one time, there was a solid state trip unit on a GE breaker that would trip when exposed to certain EMI. With some sheilding introduced by GE, it was rectified.