Well, from a purely physics point of view, before you drop any bombs the wings are producing enough lift to hold up the plane+crew plus 35000lbs of bombs. Then you drop them, suddenly(ish) lightening the plane by 35000lbs, so unless something else changes you suddenly have an extra net force of 35000lbs pushing the aircraft upwards, which is quite a lot. Would think in a more modern plane there would be some sort of automatic compensation thing going on?