I have seen discussions on this subject that goes back as far as 2013. When your in game whether in steam or Crossover, Lutris, PlayOnLinux, or Wine if your using a controller and hit the buttons just right whith steam open in the background, Big Picture opens and takes over your Display.

The discussion has been brought up to allow a way to completely disable Big Picture mode basically because it is not being used in these environments. Valve has yet to address this issue. It has been 6 years. There is no kill switch to completely disable this mode, it is always ion the background.

Your thoughts, words of wisdom, or just out flames........