As with most aircraft just before the outer marker or the FAF the configuration is changing. Gear and flaps are being extended. In heavy icing conditions the prop RPM is increased and the autopilot is disengaged. Also the Dash is not permitted to be flown on approach single engine by the auto-pilot. This must be done manually. There is also a flap drive protection that will prevent the flaps from being extended asymmetrically. Just a few factors that may or may not have contributed.