you can guarantee they found a missing nut on the accident helicopter at this point.
what they cant guarantee yet is if it was an isolated incident or not. Hence the SB and AD.
this is the data gathering stage where they find out if it's a widespread issue or not.
Ive been reading others posts about the two stages of locking. This is a must on any flight control. Dont forget though that they are only safety nets to the initial locking, which is the correct torque on the nut. Continual inspections offer the final net of safety.
In essence what I am saying is there was 4 failures here.
The torque was lost/incorrect or was never applied, allowing the nut to un-thread.
either faulty cotterpin/lockwire, or missing cotterpin/lockwire the failure was just a matter of time.
and the 4th, the inspections, didn't catch the impending loss.
Personally I think the loss of the nut would have happened quickly once torque was lost and the safety features failed to retain it.