Buffet Margin vs. Speed
 

https://cimg9.ibsrv.net/gimg/pprune....81297aab9c.gif
Looking at graphs for buffet margins (like the one above) we see an increase in the buffet margin as speed increases up to a point at which the buffet margin suddenly decreases. What is the cause for this decrease? Does it have to do with high speed flight and the loss of lift due to supersonic flow over the wings?

The same doesn't seem to happen at lower speeds.

(Note: This graph doesn't appear properly if your device is in dark mode.)

Yes..high speed buffet …on the DC 9 in the good old days of high level en route holding we selected 15 degrees of bank on the autopilot to avoid high speed buffet which happened occasionally with 25 degrees.

Correct, the buffet margin sits between the stall onset and shock wave formation at Mcrit. Hence the area of coffin corner where any change in speed or increase in load factor causes a stall or exceedence of Mmo.

Equivalent Airspeed, look it up.