So, macOS Sequioia 15.4 arbitrarily turned on automatic installation of operating system and security updates. These are things that are enabled by default on a new install—which I think is understandable. However, I had previously deliberately disabled automatic updates, and Apple decided to override my explicit actions.
Oh, and it's April Fool's Day, so thanks for that, Apple.
I don't have automatic software updates enabled because I've had Apple's operating system updates effectively brick my machine. I do a lot of music/audio work, and there's basically no telling when Apple is going to do something that will make the drivers for your music/audio hardware unusable. In fact, Apple warns you it's going to do it: most DAWs with third-party interfaces regularly see notices about how they're using a legacy system extension that will not be supported in future versions of macOS.
@geoffduncan (I still haven't made the jump from 14, and the "Update to Sequoia!" ads in the upper right corner are becoming nigh constant.)
@kiplet
I still have a machine on 10.12 because…audio drivers.