Board Thread:Speculation House/@comment-28395550-20160506235937/@comment-10380701-20160507002705

That's actually been explained. The moon did not "start" to shatter. It rotated. Just as our moon has cycles, so does Remnant's moon.

Except one half of their moon is shattered so it looks like it's slowly shattering when it rotates, before rebuilding itself again.