The formation of stable and unstable two-dimensional etch hillocks during the NH4F etching of Si(111) surfaces was observed by scanning tunneling microscopy and explained using atomistic, kinetic Monte Carlo simulations. These hillocks are kinetic, self-propagating features on the etching steps. The hillocks have a characteristic shape and size which is governed by the relative rates of site-specific etching. In simulations of highly miscut surfaces, step-step collisions lead to the coalescence and self-organization of 2D (two dimensional) hillocks into 3D (three-dimensional) hillocks. This coalescence was driven by step-step collisions which promote a "step broaching" behavior. As a result, the terrace width distribution of the 3D hillocked surfaces is exponential in form. The formation of 2D and 3D hillocks is controlled by the reactivity of a single minority species on the etching surface. Unlike previous models of hillock formation, chemical heterogeneities, such as contamination or reactant depletion, are not required for hillock formation.