The issue is that the drag is what spins the rotor, and the rotor's rotation is what makes your controls work. So when you get close to a stall, the rotor's alr

The issue is that the drag is what spins the rotor, and the rotor's rotation is what makes your controls work. So when you get close to a stall, the rotor's already going slowly, and then you push forward and it slows even further. Now you have almost no control authority and the only major force acting on the aircraft is the thrust from the propeller, which pitches you rapidly forward. I'm not sure why (given sufficient altitude) you can't cut the engine and recover using autorotation the way a normal helicopter can, but I imagine it's pretty hard to do this in a normal helicopter too if you're falling upside-down.