I have no idea either. AE is saying it is doing prerendering, so it is AE who is doing that for some purpose and for some reason. I would ask AE support to explain that. Also, perhaps the AE documentation provides some insight.
It is likely that when you are rendering the video it is set for VBR 2 Pass. VBR 2 pass renders a much better quality, but it goes through forward and then back through in reverse. When I dont have the time, or if its not a very high level production, I just do it one pass.