It's caused because of the Moon's elliptical orbit. Once per lunar orbit, the Moon reaches perigee, its closest point to Earth. And once per lunar orbit it reaches apogee, its furthest point from Earth in its orbit. Apogee and Perigee are spaced about 14 days apart. So over the entire lunar orbit, you get to watch the Moon grow small, then big, then back to small.<br /><br />Since the Moon's rotational period is the same as its orbital period, it always presents the same face to Earth... well, almost. When the Moon is closest to Earth, it is moving faster in its orbit than when it is furthest from Earth. But the Moon's rotational period does not speed up or slow down. It stays constant (ignore timeperiods of millions of years). So its rotational period gets a little out of sync with its orbital period, hence the back and forth wobble. But once per orbital period they sync back again.