Mach tuck is usually caused by two things: a rearward movement of the centre of pressure of the wing, and a decrease in wing downwash velocity at the tailplane, both of which cause a nose down pitching moment. For a particular aircraft design only one of these may be significant in causing a tendency to for example, a delta-winged aircraft with no foreplane or tailplane in the first case, and the Lockheed P-38 in the second case. Alternatively, a particular design may have no significant tendency, such as the Fokker F28 Fellowship.
As an aerofoil generating lift moves through the air, the air flowing over the top surface accelerates to a higher local speed than the air flowing over the bottom surface. When the aircraft speed reaches its critical Mach number the accelerated airflow locally reaches the speed of sound and creates a small shock wave, even though the aircraft is still travelling below the speed of sound. The region in front of the shock wave generates high lift. As the aircraft itself flies faster, the shock wave over the wing gets stronger and moves rearwards, creating high lift further back along the wing. It is this rearward movement of lift which causes the aircraft to tuck or pitch nose-down.
The severity of Mach tuck on any given design is affected by the thickness of the aerofoil, the sweep angle of the wing, and the location of the tailplane relative to the main wing.
A tailplane which is positioned further aft can provide a larger stabilizing pitch-up moment.
The camber and thickness of the aerofoil affect the critical Mach number, with a more highly curved upper surface causing a lower critical Mach number.
On a swept wing the shock wave typically forms first at the wing root, especially if it is more cambered than the wing tip. As speed increases, the shock wave and associated lift extend outwards and, because the wing is swept, backwards.
The changing airflow over the wing can reduce the downwash over a conventional tailplane, promoting a stronger nose-down pitching moment.
Another problem with a separate horizontal stabilizer is that it can itself achieve local supersonic flow with its own shock wave. This can affect the operation of a conventional elevator control surface.
Aircraft without enough elevator authority to maintain trim and fly level can enter a steep, sometimes unrecoverable dive.  Until the aircraft is supersonic, the faster top shock wave can reduce the authority of the elevator and horizontal stabilizers.
Mach tuck may or may not occur depending on aircraft design. Many modern aircraft have little or no effect.