After applying updates Windows Server will want to restart in 15 minutes if you have logged on users.

And there is no way to postpone it.

shutdown -a wont work either.

 

This is because the reboot process has been started by a service called “Windows Update”

So just stop the service from administrative command prompt by running.

NET STOP WUAUSERV

and disable the service in services snapin.

But dont forger to reboot the server manually when it suits you the best.

And for the future it could be nice to fix with proper management:

http://vnetwise.wordpress.com/2014/03/20/howto-dealing-with-windows-2012-and-2012-r2-windows-update-behavior-and-the-3-day-delay/