Well depending upon how it is setup there will be a countdown timer that if you don't reboot your pc after you get nagged a few times it will do it after 30 minutes for you. This happens only when you have it set to auto install updates. I normally keep my pc on notify only.
I've had this be a pretty annoying problem at work before. Updates gets turned on accidentally so the computer installs updates and reboots itself. The only problem with that is the computers have software loaded to revert any changes that were made, so when the come back from a reboot they want to install the updates again.