If the cast shadow of an object is moving, we assume that the object is moving and not the light source... ...A moving shadow induces a motion in depth of a stationary object (green square is not moving).
Just in case it is not yet understood. The green square above is absolutely still. It does not move. The shadow on the other hand is animated and does move across the image including changing the color slightly of the green square. This moving shadow creates the illusion of movement of the green square.
Update: Try this. Take out your ID card and cover half of the green square from the bottom left diagonal to the top right. Now it is apparent that the square is not moving. Very interesting effect.
Update 2: I just noticed something I had previously not realized. It looks like the shading of the green square changes. It seems as the shadow moves the tinting and shading on top of the green square is also manipulated to complete the illusion. The truth is not only doesn't the green square move but it is a constant non-varying shade of green. These two illusions here and here can explain why it appears as if the shading on the green square changes.