With the engine off, my car sometimes triggers a reboot/battery-icon because its highest available voltage is only 12v at that time so anything else I do (lights on, big bass hit from the subwoofer, turn on the fan, etc.) will drop the voltage down from there.

With the engine running (and therefore the alternator charging), the car's voltage is up to 14v as a baseline, giving that much more buffer space, plus the alternator is constantly charging the system.