Hypersonic speed
As a plane goes faster, the entropy change across the shock wave increases.  This results in a strong entropy gradient.  This causes air around the aircraft to swirl like a vortex, in what is called "vortical flow."  This is important because it mixes with what is called the "boundary layer" around the aircraft.  A boundary layer is a small area of air around the plane which is moving slower than the air farther away from the aircraft.  When the vortical flow mixes with the boundary layer, the boundary layer is disrupted, and the aircraft becomes very unstable.