Why Scatter fills the skip zone with weak energy that arrives after bouncing off many small irregularities in the ionosphere (or off the ground/troposphere), so the receiver hears several copies of the same signal that traveled slightly different path lengths. Those copies arrive a few milliseconds apart and add and cancel randomly, which smears the audio and gives scatter signals their characteristic hollow, fluttery, distorted sound. This is classic multipath distortion, the same effect that makes long-path and chordal-hop signals sound rough.
Watch out Blaming an unstable ionospheric region is tempting because ionization really does fluctuate, but that produces fading in amplitude, not the smeared multipath distortion; ground wave absorption and the presence or absence of the E region have nothing to do with how scattered signals sound.
Scatter = many paths at once. Multiple paths, multiple arrival times, mushy audio.