Via logansi:
The statement indicating the prop was being feathered indicates it was already shut down at the time of the separation which seems strange to me, what forces could rip a prop of if it wasn't under thrust?
Going off the news reports, they were shutting down because of a vibration.
As the prop were slowing down it would have been affected by all the other airframe/engine/gearbox/prop vibrations and may have hit a 'sweet spot' of harmonics that created enough vibration to finnish the job off.
E.g., many aircraft prop/engine combinations have an engine 'rev range' where they should not be operated for any length of time due to vibration considerations.
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