This morning, my Wordpress site went into maintenance mode without me having updated anything, and stayed there for about 10 minutes. What could have caused this? I have automatic security updates turned on, could it be that?
The short answer is yes. Wordpress does go into maintenance mode when updates are are installed. You have nothing to worry about. This is default behavior.
I do know that there are sometimes issues where Wordpress gets stuck in maintenance mode after updates, but if you don't experence such issues, you are good to go :-)