Understanding the mechanisms of structure damage under earth mass flow impact is important for building protection and geological disaster mitigation. Field observations reveal that under mass flow impact, torsion exists on bottom side columns of frame structures, and column failure leads to progressive structural collapse. However, it remains an open question to explain the mechanism of torsion generation on side columns and its effect on progressive failure of frame structure under mass flow impact. In this study, an elastic mechanics-based framework was adopted to investigate torsion generation on bottom side column under different mass flow impact. Results show that the force transfer from frontal infill wall to side column leads to unsymmetrical loading status and torque on side column. Torque causes either initial torsional failure of side column or out-of-plane bending of lateral walls which attenuates the backward bending supports. As the support from lateral wall attenuates, side column then suffers bending failure under impact, which is the triggering point of progressive structure collapse. This failure process explains the previously “complex torsional-flexural” failure mode and promotes understanding of progressive failure of the entire structure, providing guidance for vulnerability study and risk mitigation in mass flow-prone areas.