Why is it moving?
Illusions such as these fit into the “illusory motion” class. This means that elements of the picture appear to be moving when they are in fact completely still. It is believed that this is caused by tiny movements and vibrations of the eye which sends confusing information to the brain. The brain tries to make sense of this information and in the case of these illusions, interprets them as movement.
Try staring hard at a specific point of colour- notice how the image stops moving!