Retreating blade stall
From RC Helicopter Wiki
Helicopters are inherently asymetric flying machines. In forward flight, there will at any moment in time be one blade moving in the direction of flight (called the advancing blade) and one moving opposite to the direction of flight (called the retreating blade).
As the helicopter accellerates, the advancing blade's airspeed in its relative wind increases; conversely, the airspeed of the retreating blade decreases. As the helicopter goes faster, various parts of the retreating blade may stall and no longer generate lift. When this happens, the aircraft nose will rise (as the effect of the lack of lift takes place 90 degress later in the blade's cycle), possibly violently.