Flat subduction, an intermittent phenomenon along active margins, arises from well-known causes, yet the mechanisms driving its expansion remain poorly understood. The prevailing view suggests that trenchward continental motion drives slab overthrusting, causing the flat slab to expand oceanward. Here, we explore an alternative mechanism: underthrusting of the subducting plate through forward propagation of the flat-slab hinge. We directly evaluate both hypotheses through a kinematic analysis of trench and flat-slab motions using a global flat subduction database cast into multiple absolute plate motion models. Our results indicate that flat-slab expansion reflects distinct end-member processes, with forward propagation emerging as the dominant mode. We present a framework for flat-slab propagation that emphasizes the dynamic interaction between lower-plate motion and slab pull from adjacent subduction zones, an interplay validated through numerical modeling. These findings challenge conventional assumptions and underscore the need to reconsider the role of lower-plate kinematics in flat-slab dynamics.