There was a feature of Windows 8.1 Preview that opened the Start Screen on whichever screen you were actively using. This drove me nuts. I expected my Start Screen to be on a certain screen. The constancy is important and I also wanted it to appear on the touchscreen of my laptop, not my external touchless monitor.
In the final release of Windows 8.1, however, this is configurable.
Simple right click your task bar, choose properties, go to the Navigation tab and uncheck the box for Show Start on the display I'm using when I press the Windows logo key.