> Heh. Just hold your hand over part of it so you can't see the rest....

As you suggest, I think it's to do with the fact that there are so many of them, and therefore it's difficult to focus on one in particular. As you look over the image, there is an illusion of movement because the individual coloured areas are small enough and there's enough of them that it's difficult to keep track of where each one is (I think).