Infinite Loop (Barber Pole)
The stripes are moving diagonally (down and to the right). However, because of the vertical shape of the pole, your brain sees them moving vertically downwards.
The Aperture Problem
💈 Why Vertical?▼
This is a classic demonstration of the Aperture Problem. When you view a moving grating through an aperture (the edges of the pole), the direction of motion is ambiguous.
Your brain tends to perceive motion orthogonal (perpendicular) to the lines, but the boundaries of the shape (the vertical edges) strongly bias the interpretation. Since the stripes seem to travel along the longer vertical axis, the brain resolves the ambiguity by perceiving vertical motion.