Back reflection of short, intense laser pulses at oblique incidence on solid targets is explained with a model where a periodic electron density modulation acts as a diffraction grating. The pump and reflected electromagnetic waves drive through the ponderomotive force the grating and the overall system becomes parametrically unstable. The basic equations governing this system are given. A linearized stability analysis yields the instability growth rate for a homogeneous plasma and the convective gain coefficients for the inhomogeneous case. The results support the feasibility of the suggested mechanism. An absolute instability is predicted to set on a typical threshold intensity 10¹⁶ W/cm², laser pulse length 100 fs, and spot size 30 μm. The instability is shown to saturate at a level of a few percent, because the higher harmonics in the electron density modulation turn the diffraction more diffuse thus reducing both the sustaining ponderomotive force and the back reflection coefficient.