Why would you enable unattended upgrades on a desktop computer (as opposed to a server)? Is this something Ubuntu enforces?

If you don't enable this feature (which isn't enabled by default in Debian) you won't have this problem.

It was enabled by default in the standard desktop install (I didn't really customize anything I'm not a heavy user of desktop Linux). Of course I'll be disabling it next time I boot that partition. Point being Linux isn't immune to this type of annoyance.

As far as I know it is only Ubuntu that does anything similar.

Probably to deal with corruption from people applying library updates and not restarting their programs because they keep running fine in memory. I have no idea how many bug reports could be ascribed to this, but if we want a user-friendly Linux we have to put up with the safer update process. It's not lengthy at all, wait less than one minute and you're set.

NixOS and GuixSD aren't user friendly because the people who use them are like GNU/Linux users circa 2000: people insane enough to install an operating system that is very particular about who its friends are.

But they do have an excellent solution to the whole updates debacle: Install them in a separate location, initialise them when booting or when they're finished installing, and delete them when they're inaccessible from a few standard locations like /boot or /proc.

I haven't ever run into that in 8 years of running a various GNU/Linux distros (MeeGo, Elementary OS, Maemo, Sailfish OS, Debian)... though I've never run Ubuntu.

Though I’ve never run the most popular by an order of magnitude distro out there…

This also happens in fedora when applying updates through the gnome-software-center.

Let’s be honest, both gnome’s and kde’s software centers are jokes. I don’t understand why, is there no interest because everyone installs things from a command line? It doesn’t seem to be such a hard problem.