IF it is the gearboxes cracking, one could speculate that there is some other component exciting a vibration leading towards the gearbox. Thus causing stress at the gearbox.
My experience is that in mechanical construction the problem is not always the obvious.
A flat tye is not usually the fault of the tyre but usually the haphazard activity of the builder who threw the screws onto the ground. No offence to any builder.
I would make a wild guess that there is a source of vibration building up within the tail boom and tail rotor components. I could be wrong but now I've put it in print I can only now await the official report.