Stall recovery
It's reasonably simple, actually.
The stall warning system would only work within a specific range of parameters.
( An educated guess here)
The aircraft went though the range, to the "other side", ie excessive AoA, low fwd speed. When the pilot tried applying nose down, the a/c would have re-entered the operative range, hence the stall warning sounding again. Proves he was moving in the right direction, be it elevator input, or THS, (except that it sounds like the THS had not started to wind back the other way, before the ND was discontinued)
Had the ND input continued, the rollover would have continued, (with the stall warning still sounding,) until AoA reached or passed through, the "onset" parameter. We now have control.