A short circuit occurs because an ungrounded conductor comes in contact with ground or neutal conductor or another ungrounded conductor such as a double line-to-ground fault. NEC 408.36 has rules for the protection of panelboards. They state:
a panelboard shall be protected by an overcurrent device having a rating not greater than that of the panelboard
If there is a fire or some other accident occurs then the panelboard is not protected properly. The main circuit breaker is supposed to trip whenever a short circuit or overload occurs. Perhaps the available short circuit current exceeds that of the AIC rating?