The guy in the video mentioned....

Something freezing up... probably, most likely, in the transmission. That could definitely cause a massive failure causing it to break loose. That is, in fact, the only cause that seems logical to me right now BUT... like you, I don't know much about helicopters. That's why I have to consider a bird strike as 'possible' but that seems highly unlikely to me... meaning it will probably turn out to be exactly that. - JamesJM