Why does the critical angle of attack reduce with increasing mach number or What is the effect of compressibility on critical angle of attack?
First, I have to define what the critical angle of attack is:
When an aerodynamicist speaks of critical conditions, he or she means the condition when the speed of sound is reached locally. The critical Mach number is the speed, measures in multiples of the speed of sound, when the top local speed in a flow field equals the speed of sound.
The critical angle of attack is the one when the local flow speed first reaches the speed of sound. Since it depends on the initial flow speed, it is only valid for a given flight speed.
Two parameters are responsible for the local increase in speed: Thickness and angle of attack. A body needs to displace air, so the air will squeeze around it, speeding up in the process.
When the angle of attack is increased, the leading edge of a wing will stick out more and more into the flow, and again the air needs to squeeze around it. Another view of the same phenomenon is that increasing suction on the upper side of the airfoil will accelerate the air flowing towards it.
With a higher flight speed, less local flow acceleration can be tolerated before the critical conditions are reached.