A wave-front distortion of strong shock waves in gases for particular Mach numbers M has been repeatedly noted. We propose two approaches according to which there is a threshold Mach number above which the instability occurs; this Mach number is related to the unperturbed gas pressure P1. The first approach is macroscopic, and takes account of radiation; the second is microscopic, and takes account of radiation, electron diffusion to the walls of the shock tube and the non-equilibrium relaxation processes. A mechanism of four reactions is proposed. A nonlinear differential equation system is established. The latter becomes unstable for critical values of the Mach number.