Thanks to skadi's longer video, I'll offer a guess: tail rotor, or tail rotor drive, malfunction.
(aah, I see crab's got the same idea)
For Mr Dixson: would a jammed right pedal, or a servo runaway, result in something like we see in that video?
For some reason, I vaguely remember that the design of that system (unlike some others) between boost, SAS, and trim allows one to mitigate a trim runaway by disabling either SAS, trim or both.
Will see if I can find the old notebook, memory very hazy.
As I look at the first video, and mentally paste that end result to skadi's video, they appear to have been flying over a populated/built up area.