Watchdog processes don't catch everything. For example, there was once a software bug where the front panel input is locked up, and player keeps playing even after you turn off the ignition. This was one of the bugs in Beta 11. Player software is still running, so things aren't completely locked, just the inputs aren't responding (ignition sense is an input). A watchdog process would think things were OK because stuff was still running. But in that case, there was no way to get the player to reboot other than to pull it out of the sled and cause a pop.

Admittedly, that bug got fixed. I'm just using it as an example of a situation where a software bug could possibly manifest itself in such a way that it would cause the pop but a watchdog process wouldn't catch it.