Difficult to imagine how they managed to go through that sequence of extreme attitudes without anyone manging to effect a recovery.
Possible causes that spring to mind ? confusion about whether A/P engaged, as in the Kenyan & quite possibly Flash Airlines accidents / runaway stab and an incorrect attempt to offset it by elevator control which would of course be overwhelmed / A/P tripping into CWS following severe turbulence as a result of penetration of a CB and nobody noticing. . well, I am stabbing in the dark here, and still it is difficult to imagine these extreme attitudes continuing unchecked, but then so were the Kenyan/Flash accidents.