Raikum, when and where would you expect to see that evidence?
Once the plane hits the ground, it's all obscured by a fireball, smoke, and flame, or viewing other things, until that clips comes to an end.
The video extract wasn't being deliberately taken as a documentary or even directly at the aircraft. It's a bit of a luck, security truck being in the right place at the right time. As noted numerous times previously, viewing angle, perspective, movement of vehicle, and limitations from looking through thick glass (not to mention pixelation) all conspire to make it a brief and imperfect (and partial) glimpse of a mishap.
To directly answer your question: from the outside, it is a bit much to ask an observer to discern that an internal (and to him invisible) load shift forward would have directly caused a pitch down before ground impact.
What the NTSB investigation may be able to uncover, with hopefully intact FDR and CVR evidence, may provide some answers to your questions in due course.