There is a natural tendency amongst people to read visual input from left to right, top to bottom (if you are raised in a western culture, where that is the direction of writing). Things that break this flow create tension. I remember this from an example that was pointed out to me in a high school english lit class where we were watching "Deliverance": the opening scene shows dumptrucks hauling dirt from background to foreground, lower right to upper left. Watching this direction of movement makes your skin crawl for no immediately obvious reason.