We report a novel azimuthal anchoring transition of nematic liquid crystal on a polysiloxane-cinnamate treated surface under the UV irradiation. It is found, that the direction of the light-induced easy axis strongly depends on the irradiation time. Under short exposure times the orienting surface provides an orientation of LC parallel to the direction of the UV light polarization. Longer times give the orthogonal to the UV polarization orientation of LC. There is a region of exposure times that give a bistable anchoring conditions and domains with different directions of the director orientation. We propose a phenomenological model of this anchoring transition and surface bistability based on the competition between trans-cis isomerisation of the side fragments of the polysiloxane-cinnamate material and cross-linking photoreaction of the trans-form of side fragments resulting in orthogonal molecular easy axes.