Molecular dynamics simulations and energy landscape analyses are carried out for a realistic model of glassy polystyrene under shear deformation. Deformation enhances the atomic mobility, as quantified by the van Hove function. The enhanced mobility is shown to arise from two mechanisms: First, active deformation continually reduces barriers for hopping events, and the importance of this mechanism is modulated by the rate of thermally activated transitions between adjacent energy minima. Second, deformation moves the system to higher-energy regions of the energy landscape, characterized by lower barriers. Both mechanisms enhance the dynamics during deformation, and the second mechanism is also relevant after deformation has ceased. Furthermore, neither mechanism on its own could be expected to correlate all mobility data.