In granular flows, the particles not only move along the principal streamlines but can also circulate perpendicularly to streamlines slowly in a vortex-like manner. Such behaviours have both scientific and practical significances, e.g. in convective mixing of particles, but their mechanisms are still under study. Previous experimental observations have stimulated two explanations for them: centrifugal force and volumetric dilatancy. Here a theoretical model is constructed to test these conjectures, which is based on the frictional plasticity and accounts for both dynamic effect and strain-dependent dilatancy. The simulation results show that, under the influence of gravity, the centrifugal force can indeed give rise to vortices in a Couette cell. In the absence of centrifugal force, granular vortices can still be observed depending on the shape of the yield surface. If the yield surface is a von Mises circle in the deviatoric plane, i.e. of Drucker-Prager type, no vortex is observed in continuum modelling. But for noncircular yield surfaces, a clear secondary flow is generated at some boundary conditions and increases in magnitude with the non-circularity of yield surface. Large granular dilatancy occurs in some regimes of high gradients, but altogether its influence on secondary flow is not critical. The role of boundary conditions is also discussed.