What are the chances the load shifted foward and tipped the nose down?
In that case, the plane would pick up speed. Diving is converting potential energy into kinetic energy. The sum, the total energy, remains the same, apart from losses due to drag. So when altitude goes down, speed must go up.
The data in the video does not show an increase in horizontal speed while descending rapidly. Although I assume groundspeed is indicated and not airspeed, I still think at least one frame should indicate an increase in speed while diving into a headwind. Perhaps the indication is not very accurate.
To me, this suggests that the plane did incur additional drag. I think it's the normal extension of flaps and or speedbrakes.
---------- Post added at 11:48 AM ---------- Previous post was at 11:39 AM ----------
Also, when the COG moves forward, the pilot can still pull the 'stick' to correct for this. Only when the stick is fully pulled back and the nose is still dropping below the horizon, then there is a big problem. So if this was the case, I would not expect a rapid decent but a gradual one.