Some Systems Restarted Automatically after Updates Deployment with Restart Suppressed

Question

I've searched the forum and multiple blogs - but, I'm still confused about what is causing some of our workstations to restart after applying a software updates deployment.
I deployed the Dec 2013 Patch Tuesday updates to a collection of workstations. The deployment was set to:
Not allow restarts outside of a maintenance window (box is unchecked)
Suppress restarts on workstations (box is checked).
There is no maintenance window defined for any of our workstations
On all the workstations, the HKEY_LOCAL_MACHINE\Software\...\WindowsUpdate\  values are:
WUServer: http address of our SCCM SUP server
WUStatusServer: http address of our SCCM SUP server
Also, the key in HKEY_LOCAL_MACHINE\...\WindowsUPDATE\AU is:
UseWUServer: 1  (which I understand disables the use of the automatic Windows update service)
Some set of workstations automatically restarted shortly after 3:00 am the next morning. Clearly that's the Windows Update Agent jumping in a restarting the systems. However, I don't understand why it would do that. I really don't understand why it would
have occurred on only some of the workstations while others are still waiting for users to manually restart 36 hours later.
All of the workstations are in the same active directory OU - so the same set of GPOs applies to all.
Note: none of the registry keys were set directly in any GPOs - I assume the values were set by the SCCM client.
Any assistance would be appreciated.
Larry

Answer 1

depending on the OS version, this could be relevant:
http://blogs.technet.com/b/wsus/archive/2013/06/11/wsus-blog-managing-updates-with-deadlines-in-an-era-of-automatic-maintenance.aspx
http://blogs.technet.com/b/wsus/archive/2013/10/08/enabling-a-more-predictable-windows-update-experience-for-windows-8-and-windows-server-2012-kb-2885694.aspx
Don
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