Windows machines frequently do updates without a reboot nowadays.

The problem is that when an update that requires an restart is pending, it changes the standard restart button into one that does an update and restart. You can often fall into muscle memory and accidentally trigger an update. When combined with the fact that you quite often get problems that go away with a reboot you can easily get yourself into OP’s position where they’re waiting for an update to finish at a really inconvenient time.