Thats my take on it Bear. Sudden and complete LOC.
Those strapped in were incapacitated, died at or before impact, and are still strapped in. Those not restrained could not get to a seat, nor don a jacket and suffered major injuries either before or at impact. That many non-restrained (at least as many as bodies recovered) suggests passengers were not advised to strap in.
This scenario also explains why no Mayday. It was that sudden and that bad.