At high Mach number, increase of the AoA intensifies the shock wave in the wing, which in turn causes flow separation and buffet. This occurs at significantly lower AoA, than classic low speed stall - think ~5 deg AoA instead of 15...
P.S. What airplanes are we talking about? Cessna 150, or jet transport. The former rather doesn't suffer too much from compressibility effects...
