Constant MAP assumes constant power. With increasing forward airspeed the blade section angle of attack reduces. If the blade angle remains unchanged the RPM would therefore increase. To maintain the RPM the blade angle (pitch) must be increased thus increasing the torque and slowing the propeller to its original set RPM.

For your information fine pitch is associated with high RPM and low forward airspeed. Coarse pitch with low RPM and high forward airspeed. Blade angle of attack is determined by RPM and TAS.