It is because our human brain registers a change in light (like the cube flashing from light to dark) as motion, that it looks as though the cubes are moving when in reality they are totally static. The video was also shared by a user on Twitter Steve Stewart Williams.

It is a simple wire-frame, two dimensional drawing of a cube with no visual cues as to its orientation, so it can be interpreted to have either the lower-left or the upper-right square as its front side.