Windows 8.1 Preview was officially retired yesterday, so all machines that are still running it right now are automatically restarting every two hours.
Microsoft wants all users to move to the full version of Windows 8.1, as it provides several
performance improvements over the preview build, as well as enhanced functionality that makes it more helpful for users worldwide.
Windows 8.1 Preview was launched in June 2013 at the BUILD developer conference and was supposed to provide us with a short glimpse into the feature line of the first update for Windows 8.