Repeated shadows of a cloud such as the one in the video are formed when a shadow is cast upon several layers of thin dust or haze. The smaller, darker, closer-in shadow likely lies on a layer that's only slightly hazy and is visible in contrast because the air surrounding the clouds is somewhat more hazy and so scatters sunlight more strongly -- as though the air itself was brighter than it actually is. The larger, dimmer shadow is cast on a more distant haze or dust layer. There's even a faint third shadow further out.

Technically, this is referred to as differential scattering cross section, and it's a strong function of scattering angle. You're more apt to see such shadowing only when look toward the Sun, usually within about 10 degrees of it.