Gosh, they went over this in the Keys to the Kingdom tour but my memory is a little vague on the specifics. I do remember the cm saying that the parade during the day goes in one direction but the one in the evening goes opposite. I'm pretty sure the evening parade starts in the front of the park--the idea being that if it ended in the front on Main St. it would create a mass exodus of people bottlenecking the exit. But if they have the parade START at the front and end further into the park, it will help to spread out the masses of people a little more evenly and it will give the parade-viewers a little more time to walk to the front. There was a reason for why the opposite direction is better for the daytime parade, but I can't remember what it is.
Sorry I don't have details, but I hope I've helped a little bit.
-Laura