How do you know what their 'arousal level' was? I can see two people in the back on dispatcher harnesses who are sitting in their respective doorways. You still haven't given a clue as to how you actually know what else was going on in the aircraft, apart from supposing an 'outbreak of dullness' from watching a distant video clip. You obviously have more intuition than me to see what was happening in all the crew positions (four is it?), on the intercom, on the radio and their state of alertness (or arousal). Maybe I need a new P.C., which can see these things for me too!
All I think I can see is that a sudden yaw input, probably without any warning to the crew, put the tail over the edge, probably too quickly for the 'f*ckwit' in the left doorway to say anything to stop it.
Maybe a proper board of enquiry will determine the facts and dish out the appropriate punishment(s) rather than someone supposing an outbreak of 'dullness' being the cause.