Identifying shutdown cause

This morning, I sat down at my Mac (which is never powered off) to find that it rebooted itself this morning at 3:30am.

In trying to figure out why, I ran across this wonderful article:

It describes use of the log show command to determine why the system had shut down, which I believe is already well-known (although I hadn’t heard of it until today). More useful, it includes a list of shutdown cause codes (updated for Big Sur).

In my case, the shutdown/reboot was due to cause -20: “BridgeOS T2-initiated shutdown”. So something running on the T2 chip caused the shutdown. I don’t know if the T2 crashed or if it decided to send a shutdown command to the rest of the system for some other reason.

Unfortunately, I don’t know if there is any kind of log file from the T2 (or if I would even have access to it if there is) that might tell me why it rebooted my system at 3:30 this morning. There was nothing significant in /var/log/sytem.log at that time - just the usual set of messages that get generated when the system reboots.

I did notice quite a lot of log messages in /var/log/install.log at that time. I’m wondering if Apple pushed an update that caused a reboot (maybe an update to the T2’s BridgeOS), but I’ve never heard of that happening before, especially since I have automatic updates disabled on my system.

Anyone here have a clue?

I do not have a clue, but thanks for sharing. I had two reboots during the night 3 or 4 weeks ago and guessed it had been a power outage. Next time I will check with the “Previous Shutdown Causes Explained”. This is a 2010 MacPro, so no T2 chip.
Some years ago I experienced the same several times a week for some time before I found out it was the cleaning staff that removed the power plug to use it for their equipment.