Should the aircraft had few more knots to loose(sic) before reaching its lowest speed this AP disconnect and pitch down movement would have happened later, closer to the ground, with no more room to reduce the high Vz.
The autopilot disconnect was caused by a control input. From the report found
here:
At 240’ the aircraft commander selected flap 25 in an attempt to reduce drag. As the autopilot attempted to maintain the aircraft on the ILS glideslope the airspeed reduced and by 200’ had reached 108 kt. The stick shaker activated at approximately 170 ft, and shortly afterwards the First Officer made a nose down pitch control input which reduced the aircraft pitch attitude and caused the autopilot to disconnect.