@westcoastaudiophile is correct. 50A transients in a 400wpc amp without soft start circuitry seems pretty normal.
The duration is in the milliseconds though, so nothing has time to overheat and no fuses blow.
Available short-circuit current (ASCC) is the measure of how much current a branch circuit can deliver through the breaker during a bolted fault condition (aka a dead short). For 14AWG wire it is if I’m not mistaken >500A.

