Recent measurements by Electric Capacitance Tomography (ECT) reveal that, under certain operation conditions, the solid concentration distribution in the dense and acceleration regimes of gas-solid riser flows exhibits a strong heterogeneous structure within the same cross-section. This core-annulus-wall heterogeneous structure, significantly different from the commonly-known "core-annulus (wall)" two-zone structure in riser flows, becomes unstable with the increase in solids loading, which eventually leads to the occurrence of choking . In this study, we propose a mechanistic model to investigate the formation mechanism of these interesting heterogeneous structures. Our model simulation shows that the commonly known "core-annulus (wall)" flow structure occurs at low solid loadings and/or high gas velocities whereas the core-annulus-wall flow structure is formed at moderate solids loading and/or at low gas velocities. With a high solids loading at low gas velocity, sever back mixing results in a solid concentration peak formed in the core, which not only causes the flow instability but also possibly triggers the choking. Model simulations are validated by direct comparisons against measurements in solid concentrations as well as pressure drop along the riser, which shows a fairly good agreement.