You were likely nagged for weeks by the system to update OR you dual-boot Linux and you switched over to Windows for a quick document edit. And then, when the p

You were likely nagged for weeks by the system to update OR you dual-boot Linux and you switched over to Windows for a quick document edit. And then, when the popup showed up, and gave you the choice to dismiss it, you didn't pay attention (who pays attention to popups?) and clicked the "reboot now" button. It happens. But, now that you know what to avoid, you won't run into the issue again. Assuming you ever use Windows again, that is.