Big parts on Mag Plug

what are the main causes of gear box damages?

i think  the following....

1.out of balance propeller, 2.carbs out of sync,
3.Low friction torque on the gearbox due to weak and or worn Bellville washers, (this is supposed to be checked every annual or 100 hrs. whichever comes first)
4.Bad engine mounts,
5.wrong oil,
6.Improper starting and stopping techniques,
7.failing to get the gearbox serviced at 1000 hrs. (less if all you use is 100LL)

you are also add some more points if you think the story happened...

And improper idle operating technique.  This engine is mounted on a seaplane.  Which of course has no brakes available when being operated in the water.  Rotax says the engine idle may be set at a lower RPM, but that pilots must try and operate the engine at a higher idle RPM whenever possible to prevent gearbox wear and/or damage.  This would also apply to a Rotax equipped plane on snow skies.