This is a well known problem with gyrocopters. The first answer was partially correct in that the problem was caused by pushing the stick forward and unloading the rotor. However, the problem wasn't a slowing down of the blade. The actual problem was that many of these gyrocopters would tumble under these conditions. If you were close to the ground you would crash before you could recover. And, even if you had altitude the tumble could overstress the rotor and a crash would still occur. The biggest problem though was that no one seemed to know why and since those who did tumble didn't usually survive there wasn't anyone to ask. So, if you flew a gyrocopter the first rule was to never push forward sharply and unload the blade.
Eventually, people who understood aerodynamics and physics got involved. They studied gyrocopters and realized that it wasn't a problem that was inherent to the type. The actual problem was that many of these had a thrust line that was above the center of gravity. In the small gyros the pilot's weight accounted for a lot of the loaded weight and the pusher configuration meant that the pilot's seat blocked the airstream. So, there was a incentive to raise the engine both to get clearance for a larger propeller and to get the blade up into clean air.
The A-10 Thunderbolt II has a similar problem. The engines are placed above the fuselage so that the hot exhaust goes over the horizontal stabilizer where it is blocked from view of heat-seeking missiles on the ground. This high thrust causes the A-10 to pitch downward. The solution was to angle the engines downward slightly which would normally make the plane pitch up. So, these cancel out.
So, anyway, the high thrust line isn't usually a problem on gyros since the main drag is from the rotor which is even higher. However, this doesn't work when you unload the rotor. Unloaded, the rotor drag goes to zero and free-body physics takes over. If the thrust line is above the CG then the aircraft will rotate forward and if it rotates far enough you will tumble. The only solution is to lower the thrust line so that it passes through the CG. This can make the aircraft pitch up more since the drag from the rotor is still high. You can try to counter this by angling the thrust line up but this may put it back above the CG. And, the thrust line might still be above the CG if another lighter weight pilot flies.