So it was a chuffing big boat, in daylight, in reasonable sea and wind conditions with crews appropriately trained and practised - what went wrong?
Since the report states the winch wire snagged a mast, you can rule out mechanical failure of the winch - it won't hold up several thousand tons of merchant vessel and the wire appears to have snapped after getting snagged.
There is no mention of a mechanical failure of the aircraft either which pretty much leaves operator error due to either the choice of the winching area or the control of the winching evolution or both.
Sorry to be criticising the crew but there seems little else that could have caused this unfortunate accident other than poor piloting, poor winch operating or both.