From what I was told, the way the parades go in the MK is to do with crowd control. Share A Dream for example starts at Frontierland, near Splash Mountain, and finishes near the entrance to the MK on Main Street, however, Spectromagic goes in the opposite direction, why, well think about it. As soon as the parade finishes at night, people want to get out of the park, if the parade was to finish on Main Street, there would be loads of people pushing through the crowds still watching the parade, and jumping in front of the floats. Because they would have come from the back of the park (well, Frontierland area) where the parade has already finished. By operating the parade the other way round, the park will start emptying from the front of the park. In the daytime, the people watching the parade in Frontierland can disperse into the middle of the park, while the parade is still running on Main Street.