What is watchdog - a bit of background (very simplified, can't remember the exact mechanism).
When shutting down, systemd by default sets a watchdog timeout of 10 minutes, then closes the watchdog before executing systemd-shutdown. If systemd did not stop the watchdog, Linux warns a user. This has the effect that if shutdown is not completed within 10 minutes, the hardware watchdog will trigger the specified action. Which is usually shutdown.
How to disable 10 minutes watchdog timer.