This question already has an answer here:
I haven't been interacting with Mail for half an hour, but suddenly it opens its main window in split screen with the fullscreen video player that I'm watching. This happens every other day.
I can't really say if this only happens when Mail is running, since it's hard to check that after the fact. Assuming it only happens when Mail is running, can something be done other than making the habit of always quitting Mail as soon as possible? Is there some hidden feature triggering this?