Go into your preferences and uncheck the monitor that you don't want to go full screen.
It happens to me as well ...when Premiere is minimised..
I just click on the second screen if I see it.
Its annoying and I suspect it is triggered by "Auto save" occuring in the background... and that focusses Premiere.
I'll do that; but that doesn't change that it shouldn't happen, or why it happens. I agree that it may have something to do with autosave.