As an aircraft accelerates through the transonic speed range, shock waves form on the upper and lower surfaces of its wings.
A shock wave is a sudden increase in static pressure. As the air flows through the shock waves, this higher pressure compresses it. This sudden pressure increase causes an equally sudden decrease in the velocity of the air. This in turn causes the boundary layer to separate and become turbulent. The separation of the boundary layer causes a large reduction in lift on the area of wing directly behind the shock waves. The turbulence caused by this separated airflow also cause buffeting.
The separation of the boundary layer causes a large reduction in lift and a large increase in drag on the area of wing directly behind the shock waves.
The loss of lift over the rear area of the wing initially causes the Centre of Pressure of that section of the wing to move forward. In the case of straight winged aircraft this will tend to cause the aircraft to pitch nose up.
If the aircraft continues to accelerate, a second shock wave forms under the wing and both of these shock waves gradually move aft to the trailing edge. This causes the Centre of Pressure of that section of the wing to move aft. This aft movement of the Centre of Pressure tends to cause the aircraft to pitch nose down.
The shock waves form first at the points at which airflow velocity is greatest. This occurs at the wing roots because this is the area where the thickness of the wing and hence acceleration over it is greatest. This means that the shock-induced separation and loss of lift will occur on the upper surface at the wing roots.
In the case of swept-back wings, the roots are ahead of the outer wing and wing tips, so the initial loss of lift is on the forward part of the wing area. This causes the overall centre of pressure of the whole wing to suddenly move aft, causing the aircraft to pitch nose down.
Prior to the onset of shock waves, the lift generated at the wing roots will produce downwash, some of which will flow over the tailplane. This increases the down force generated by the tailplane and in so doing contributes to the correct trimming of the aircraft. But the sudden loss of lift caused by the formation of shockwaves on the wing roots greatly reduces this downwash. This in turn reduces the down force generated by the tailplane, thereby upsetting the trim of the aircraft. The combined effects of the rearward movement of the C of P and the loss of downwash over the tailplane cause the aircraft to pitch nose down.