Simplistically, subsonic, your lift is generated by the wing and control surfaces (tail) and effectively generates a constant moment about the wing ~25% MAC (mean aerodynamic chord.) Supersonic, everything in the breeze generates lift based on surface area and AOA (including the fuselage.) The wing now generates its lift around a 50% MAC point which is considerably further aft than 25% MAC and thus there isn't enough down force from the tail at its subsonic position to keep the nose from falling through. If you trim the whole horizontal stabilizer to bring the pitch moments back in balance and succeed in bringing up the nose and the aircraft then decelerates to subsonic, there will then be a pitch up effect.