I know that burgers are usually stacked to tilt away from the camera in photography to show the contents. (ie the bottom bun is laterally closer to the camera than the top in a downward view) I don't know why you would stack them to the side because it's more obvious, and in this case you can hardly see anything different at such a shallow angle. It's almost like they stacked them and then took the picture from a randomly selected angle or something.