The Fairford crash was (I believe) proved to have been the throttle slotted instead of the Condition Lever.
This, instead of feathering the prop and closing the HP fuel valve, would have withdrawn the low pitch stop and allowed the prop into the ground range in flight (perhaps even reverse pitch).
Obviously, on MCT, the aircraft is at circuit speeds and heights. Putting an engine into the ground range (and SOP dictated, an outboard was used) in flight would generate such asymmetric forces on the aircraft that it would render it uncontrollable.
As I said earlier, provided we learned lessons, these tragedies probably saved more lives than were lost in each accident. We ignore such lessons at our peril.