My understanding is that 80% was to allow the aircraft to descend below DA/DH to perceive a more accurate visual environment, before disconnection.
Also, we used to mandate that if a manual landing were to be performed (the aircraft doesn't know if it Cat III or CAVOK), then the auto-pilot must be disconnected by 80' (80%?). This was a legacy Airbus requirement applied to both A/B fleets and eventually removed.