Have ya'll called Bell to see what they have to say about it?
No doubting it happened or if it was an eye opener....just wondering what the actual cause of it is?
My hunch is something to do with Collective Friction or a bad collective bell crank or bearing.....everything all ticketyboo with the bellcranks, linkages, rod ends, collective friction setting? Any elongated bolt holes in the Bell Cranks?
If it were the friction setting....it should also show up in the AP mode or something similar.
Was the collective itself moving?
Did either pilot brace the collective with a knee so it could not move?