A thrust reduction does not mean an airspeed reduction... if you idle the engine and put your nose down does the airspeed still drop off? As airspeed drops off, for you to maintain straight and level you must pitch upwards... it is this upwards pitch that will then, eventually, cause the wing to exceed its critical alpha (around 15 degrees on a clean wing, less with more trailing edge flap) thus the wing produces no lift. The aircraft will then do as designed and pitch downwards.