Default Address Book is changing from GAL to an Address List

I have an Exchange 2010 server.  Clients are a mix of Outlook 2007 and Outlook 2013.  Server has been in place for a few years now.  When the server was installed, I created individual Address Lists based on locales.  Up until several weeks ago, the default Address Book has been the GAL.  Now, the default Address Book for some users has changed from the GAL to one of my created Address Lists.  I can manually change it back per user by going into the Outlook Address Book settings.  

What would have caused this to happen and how can I change the default back for all users without going to each workstation one by one?
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Raymond PengSystems EngineerCommented:
No existing GPO to do this.

This would need to be  accomplished by creating and modifying a new GAL. The steps can be found below.
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