I guess it is the same with similar optical illusions: the lens of the eye is not flat (obviously) and therefore every straight line will be bent when hitting the retina. The fascinating “software” of the brain of course corrects these things into straight lines. (btw, your smartphone camera software will do the same.) But now there are no straight ones. What to do? Not only this, when moving your head / eyes the lens moves relative to the image. Of course focusing on another part of the image makes the lens refocus which causes it to deform causing again shifting of the lines. And when something is moving, even only seen in the corner of the eye, it will try to track that movement which causes another refocusing. In photography you get line distortions depending the focal length. Especially with lower focal length (wide angle). I believe that this page gives you a short impression on what line distortion is: https://clickitupanotch.com/lens-distortion/ I hope my explanations are close enough to the real phenomenon behind this optical illusion.