From this answer we learn that an increase in flow speed is always coupled to a decrease in density. The ratio between both changes grows with the square of flow speed and equals unity when Mach equals the inverse square root of the ratio of specific heats.
Why is this important? For an airplane to fly through air, this air has to make way for the airplane. At subsonic speed this happens by an increase of local flow speed while at high supersonic speed the air slows down, getting denser. In both cases the air will temporarily occupy less volume so there is space for the airplane to pass.
Think of the air flowing past the airplane as flow through tubes. In order to make way for the airplane, those tubes must shrink in diameter around the airplane so it can fit in. They will do so at subsonic and high supersonic speeds, but much less around Mach 1. That is the reason for the maximum in the drag coefficient between Mach 1 and 1.2.