From SKYbrary Wiki
Mach tuck is a nose down pitch tendency due to a change in the position of the centre of pressure resulting from a rearward movement of the shock wave which occurs as an aircraft in transonic flight accelerates beyond its limiting mach number (MMO).
As an aircraft accelerates, the aerofoils create more lift. To maintain level flight, nose down trim is required. If the aircraft is in transonic flight and continues to accelerate, the resulting shock wave that forms on the wing moves aft and becomes stronger. This results in a rearward movement of the centre of pressure which causes a nose down or "tucking" tendency referred to as Mach Tuck. If the aircraft is allowed to continue to accelerate beyond the limiting mach number, the centre of pressure may move so far rearward that there is insufficient elevator authority available to counteract the nose down moment. At this point, it is possible that the aircraft could enter a steep, potentially unrecoverable dive.