Your eye movements make this design seem to shimmer.
You can get a stronger shimmer effect by moving your head as you stare
at the design.
The motion you see is not in the design, but in your own eye. Even though you're not aware of it, your eyes are constantly making small, jittering movements, which continually refresh the image cast on the back of your eye. Normally, your brain can ignore this motion, so your picture of the world stays stable.
Each time your eye moves, the old image is briefly superimposed on
the new image in your eye. When your eye moves over the repeating, evenly
spaced lines of this design, the old and new images are superimposed
to create a swirling shimmer.