The static prop load is larger than the load the engine sees in flight (see my post in Electric Super Clown thread in the Electric forum--where I plot the motor current and rpm thru a pattern). Also the cooling is less. When you pull the nose up, the mixture leans, and I am guessing now that that puts the engine over the edge.
In flight the prop load is less, the cooling is greater, so even when the nose goes up, and the mixture leans, the better cooling and lower prop load keeps the engine from going into a lean 2 stroke.