I think part of it is that Summer seems to be for the action/adventure/drama type stuff, "guy" movies and some "date" movies, and near the holidays seems to be concentrated more towards "family" movies, and many "high fantasy" type stories are often considered "family" movies to varying degrees.
The biggest examples would be Harry Potter, Narnia, and the Lord of the Rings (yea I know the LOTR has some rather non-family stuff in it, but still). The major fantasy franchises of late have been primarily winter release.
And I *do* think there is a bit of an association with "high" fantasy and fall/winter, even in the works themselves...Lord of the Rings really gets going in the fall, and ends in the fall...the Dark is Rising sequence has some of that too...