Was the PNF simply reacting instinctively to a sudden pitch down of the nose?
Only the PNF himself is in a position to correct you. PPRuNers can only speculate. A note in the report says the airplane encountered a head-on gust of 25 kt, followed a few seconds later by an upward gust of 35 kt. The airplane probably pitched down in response to the upward gust. The PNF's instinctive reaction may well have been in response to the combination of overspeed warning, g-force and pitch-down movement.