Microsoft offers critical update for Windows Phone 8.1 Preview for Developers program
If you're in the Windows Phone Preview for Developers program, you should go to Settings > Phone Update > Check for Update to see if you can coax the update out of the server. The Preview for Developers program gives members the ability to see Windows Phone OS updates before they are sent out to the public. For developers, this gives them the opportunity to make sure that their apps run smoothly on a new build of Windows Phone.
In other words, despite the name of the update, this appears to be nothing more than some bug fixes. Still, it seems like something that Microsoft had to take care of so that it could make sure that its update system is working properly.