Other than unforeseen surge in draw, what can cause breaker tripping?
The 100 amp breaker could be faulty.

The wiring between the two panels could be faulty. A break in the wire could cause resistance and draw enough current to trip the breaker. Or a break in the insulation could allow a short.

If a breaker in a subpanel is faulty, and it fails to trip when the circuit it protects draws too much current, then the upstream breaker will trip. In your case, the upstream breaker is the 100 amp breaker in the main panel.

There is also a small possibility that you have the wrong 100 amp breaker for you application. The breaker for your AC might have a trip curve that allows for high current draw briefly, but the 100 amp breaker trip curve does not. Please post the model numbers of those breakers.