Substrates with a high in-plane symmetry (threefold or higher) induce multistable anchorings in nematic liquid crystals: The preferred nematic axis in the bulk can take several equivalent orientations with different in-plane directions. To elucidate the mechanisms leading to this degeneracy, we have investigated the relation between the orientational order of cyanobiphenyl molecules at the surface of phlogopite mica plates (exhibiting a threefold symmetry) and the observed bulk orientations induced by these surfaces. In particular, in-plane bulk reorientations were observed when the system was put in the presence of water vapor. These appear to be driven by changes in the tilt distribution of the surface molecules. To account for our observations, we propose an anchoring mechanism based on the coupling of the uniaxial bulk nematic order to the surface-induced threefold order present in the first molecular layer. For this purpose the Landau-de Gennes theory of nematic liquid crystals is extended to take such a coupling into account. The calculations show that the observed in-plane bulk reorientations can be induced by changes in the surface value of the scalar nematic order parameter.