The main reason for the drop in MAP is that if you change airspeed at the same collective setting, you're changing the induced velocity through the disk, and hence changing the drag on the rotor blades.
The engine is there to overcome the drag on the blades (lift being a very useful by-product).
If you fix the collective, start at 60 knots, and increase airspeed to 90 (in a dive naturally), you'll see the MAP/Torque decrease. With the same collective position slow to 20 knots or so, and you'll see a torque/MAP increase.
Same thing happens when going through translational lift- at the same time that you see a large increase in 'performance', you'd also see a decrease in torque/MAP when you didn't move either the collective or pedals.