Are you often have connected an external screen to your computer and are working with SAP PI? I have a screen that I often connect to. If I put the Integration Builder or ES Repository on the screen that is being disconnected, the application will start on that screen next time. You cannot move it back to the main screen with the mouse.
There is a simple trick to make this happen. Hold the ALT key, press the Arrow Down and until Move is selected. Then you can move the Window back to the window that you have. You can see how this works on this video.
If you find it helpfull please leave a comment.