Likely it is a little off-balance and not carved perfectly, so the people walking by are causing it to vibrate and "right itself". It doesn't seem to move when it's daylight and there's no people walking around.
Bascially the base is slightly convex and not perfectly symmetrical. This makes it into a "rattleback." Vibrations from the patrons walking by are transformed into rotation.
Into the Abyss I'll fall - the eye of Horus Into the eyes of the night - watching me go Green is the cat's eye that glows - in this Temple Enter the risen Osiris - risen again