I'm not an expert, but I believe I have experianced the same thing with my Morris the Knife. If I'm not quite giving it enough throttle, the nose suddenly will drop to one side slightly and it will do a rapid 1/2 roll. Like you, I was very supprised when it first happened; it doesn't seem to make much sense. Anyway, stay on top of the throttle. As soon as you reach 0 air speed, go to the throttle position for a good hover.