AC, talked to some of my Vietnam era RAAF Huey instructors and the only thing we can come up with regard to the nose over is the note in the flight manual.
Should the engine malfunction during a left bank maneuver, right cyclic input to level the aircraft must be made simultaneously with collective pitch adjustment. If the collective pitch is decreased without a corresponding right cyclic input, the helicopter will pitch down and the roll rate will increase rapidly, resulting in a significant loss of altitude.