This photo was taken at the British Museum in London. It shows that not all background lines are horizontal. Chances are the central piece is done and the background is left to assistants to fill in. What determines their line, most often, is the position of the mosaicist. We naturally set tesserae in the the same way we write, in lines horizontally across the body. Where you get these lines going in different directions (some vertical, some horizontal) it shows that you had more than one mosaicist setting the background and sitting in different position to each other.