Years ago, I lived in NYC, and my roommate was a director of photography for National Geographic, and various other nature documentaries. I loved photography (still do, but much less time for it as a late 30s adult than a mid 20s adult), and she was kind enough to answer any question I had regarding film/photo.
She told me that "left to right" denoted progression in the story, "right to left" told the viewer the subject was "exiting" the current scene.
She didn't go into the details of WHY, and I probably didn't probe deeper, but it stuck with me, and I notice it all the time in film and television.