Have you had a client want to throw their computer out the window because Windows 8 rebooted right in the middle of their important conference call for no apparent reason? And they wonder, weird, I had my computer sitting idle all night doing nothing, why didn’t it reboot then?
Well, some idiot at Microsoft designed it this way. The thinking was something like, ‘I guess we don’t want to reboot at 3 AM in case something is open, so we’ll wait until 3 days after updates are installed, then start annoying the user with a restart timer once there is activity on the PC, which will force a reboot if ignored.” The hilarious thing is, this affects 2012 Server. So admins who enabled automatic updates suddenly find production servers rebooting in the middle of the day! Nice job, Microsoft!
There is a fix. In 2012 R2 and 8.1 there is a group policy setting to change this behavior-
Computer Configuration\Administrative Templates\Windows Components\Windows Update\Always automatically restart at the scheduled time
On windows 8 and 2012, you need to install the update rollup http://support.microsoft.com/kb/2883201
By default this behavior will still exist though unless you change that GPO.