... the wrong grease being pumped into the main rotor
Standing by to be corrected, but I think it was the wrong anti-seize compound/thread lubricant applied to the m/r hub nut threads. I think it reacted with the xmsn oil vapour and attacked the threads causing them to fail.
Economic I'm sure but what a shame to miss out on what could have been a decent ferry flight
It went as a part-load with a load of other stuff for the film.