It is a combination of several effects:
Aeroelasticity: With the higher forces at high speed, the structure deforms such that the effective flow angle at the tail surface is reduced.
The supersonic lift curve slope of the tail surface decreases with Mach while that of the fuselage stays roughly constant, so with higher Mach the tail contribution to ...
Here you see a fuselage viewed from above. The horizontal line at the right end of the ellipse is the tailfin. On top is a subsonic plane, and at the bottom is a supersonic one.
The thin lines are pressure, the thick ones are forces, and the arrows show flow direction and magnitude.
As you can see, at subsonic speeds suction is more dominant, ...