The interaction of magnetized edge plasma with the material surface in the divertor region of a fusion device is analytically investigated. It is shown that the process of surface erosion is unstable against small perturbations of the surface shape, which leads to a gradual roughening of the initially smooth target surface. Two distinct reasons are studied for possible drivers of such erosion instabilities: modification of ion deposition flux and surface temperature variation. The wavenumbers of the unstable perturbations are determined from a linear approximation of the surface erosion equation. For the plasma parameters relevant to fusion reactors the estimated mean time of surface roughening is about a few hours. Consequently, the erosion instability is of importance for long pulse and steady state operation of future reactor-grade fusion devices