Vortex-induced vibration is a representative wind-induced response of long-span bridges, comprising Karman vortex type vortex-induced vibration and motion-induced vortex vibration. Although the former has been extensively investigated, research on the latter remains comparatively limited. To address this gap, this study investigates the torsional mode of motion-induced vortex vibration in a 4:1 rectangular section using large-eddy simulation (LES). A mean-subtracted conditional averaging technique is proposed to elucidate the evolution of key vortices, and the underlying mechanism governing the flow field and aerodynamic moment are systematically analyzed. The results show that the downstream drift of leading-edge separation vortices along the section surface, together with their subsequent merger with trailing-edge vortices, constitutes the primary source of dynamic excitation that triggers motion-induced vortex vibration. Furthermore, the kinematics of the vortex structures exhibits strong spatiotemporal coherence with the distribution of aerodynamic moment density. The dynamic evolution of the flow field structures not only alters the amplitude of the aerodynamic excitation moment acting on the section but also modulates the phase relationship between the aerodynamic moment and the torsional response.