Why The Moon is a rough, irregular reflector, and its libration (the slight wobbling of its face as seen from Earth) constantly changes the relative path lengths from the thousands of scattering points on its surface. Those reflections add and cancel at random, producing rapid multipath interference on the received signal. The result is the fluttery, irregular fading EME operators hear, sometimes fast enough to make CW sound rough.
Watch out The choice about the echo returning several hertz lower in frequency describes Doppler shift from the Earth-Moon relative motion, a separate EME effect; a slow pitch change would also be Doppler, not libration.
Libration = the Moon wobbles, so its rough face scatters many echoes: flutter. Doppler = frequency shift, a different problem.