  We propose a two-stage mechanism for the rotational surface disordering phase
transition of a molecular crystal, as realized in C$_{60}$ fullerite. Our
study, based on Monte Carlo simulations, uncovers the existence of a new
intermediate regime, between a low temperature ordered $(2 \times 2)$ state,
and a high temperature $(1 \times 1)$ disordered phase. In the intermediate
regime there is partial disorder, strongest for a subset of particularly
frustrated surface molecules. These concepts and calculations provide a
coherent understanding of experimental observations, with possible extension to
other molecular crystal surfaces.
