Its easy to reproduce.
Open the level blueprint tab, then return to the scene view.
Play the project.
While the project is running, switch to the second tab.
Stop the project.
Right now, I want the editor to remain at the same tab, as I am either debugging and found the problem, and want to fix it, or want to develop it at that tab. I HATE IT when the editor switches the tab on its own. A program should never make decisions for me, especially when its a matter of where my focus is, as a developer.