When Windows writes files it will use the first free space it encounters & as a movie is being created the file(s) will grow & the fie system will then allocate the next available free space & so on.
So if the defragging program just defrags the individual files that are fragmented without moving files into the first empty slots then a large number of tiny pockets of free space will be left & these are the first filled.
The application I use (PerfectDisk) has a Smart Placement option which moves all the files into the first available free slots so all that is left is large contiguous areas. In this situation what you're seeing won't happen.
That's a rather simplistic explanation but it should give the idea as to what's happening.