It didn’t ask for their sim PIN, which is expected if the sim does not lose power, like in a full reboot without power off (eg kernel panic auto restart).
This is not to be confused with iOS unlock. SIM PINs are not commonly used in the USA.
Some reboots on iPhone (including if you force it yourself by holding HOME + Power on iPhone 6) don't relock the SIM card, so that might be the "PIN" that was missing.
That's because the iPhone didn't reboot fully. This doesn't deterministically cause a panic, sometimes it just takes down parts of the system.