This paper aims at an explanation of the instability of etch fronts of single crystalline material. This instability manifests itself e.g. when  oriented single crystalline silicon wafers are etched in aqueous KOH solutions by the formation of hillocks, and by the formation of terraces when etching different crystallographic orientations. The key for understanding is the observation that the faces of the hillocks are composed of stepped  faces. The steps emerge from the edges of the pyramids. A comparison of the step speed and the rate of step formation naturally leads to a criterion for stability of the hillocks. We obtain good agreement with experimental data for silicon etched in KOH solutions.