That really is WEIRD!!! I would swear it's an animated image, but if you cover one eye and stare at a single spot, everything stops moving!
Methinks this has something to do with the fact that most people have at least a touch of astigmatism. Here's a simple test:
Look at the image on that page. If you see dark and light triangles or spokes, or if it looks like the circles are rotating, you've got astigmatism.
Years ago there was a company that ran magazine ads with that circle image and the headline "If you see triangles or motion in this image, you've got astigmatism and should come to us for an eye exam!" Of course they didn't mention that your vision is probably "normal" (20/20) even if you do have astigmatism.