The fact that droop and flap had separate selectors moving in the same quadrant was undoubtedly a factor in the PI accident.
One of the more ludicrous "safety" features of the design was that they both had aerofoil shaped handles at the top of their levers - but the droop had a slight dip at the leading edge and the flap had a slight dip at the rear. How many actually examine the shape of a handle before they operate it?
On the Trident 1, the other lethal factor was that selection up of the leading edge device caused an immediate large increase in stalling speed. This was rectified for the later marks.