How can I find the reason for my Mac's recent "restart"?

My Mac just "restarted" while I was looking away and I'd like to know if it was a crash or a postponed restart due to an update. My Mac does sometimes crash and restart but hasn't done so for a couple of months. I installed some OS updates in the past couple of days but chose not to restart yet. Google Chrome was getting slow and had been displaying the green update arrow for a day or two. I clicked on the update button and wandered off to do something before it responded in any way. When I turned back to the computer it seemed to be "restarting". I'm using the scare quotes because I don't think it was rebooting from scratch since I didn't see the Apple logo with progress bar, which can be a little slow on my system. I verified that it had restarted by switching to the Terminal app which showed the Restored text with a timestamp from about a minute ago. When I check when the last *reboot* was using uptime or System Information it tells me 32 days. The App Store is telling me that I still need to restart to complete updates. ---- So how can I check why it just "restarted"? I want to know if my crashing problem came back after such a long time of stability, if asking Chrome to update somehow allowed the other pending upgrades to restart the OS, or if it was some other reason.