Metal-induced layer-exchange crystallization (LEC) of amorphous silicon enables large- grain polycrystalline silicon films below 550◦C, yet the fundamental driving force for sponta- neous layer inversion has remained elusive for over 25 years. We propose a novel mechanism in which both Si–Si and Metal–Metal atomic pairs initially reside in the repulsive branch of the interatomic potential because the non-equilibrium bonding parameter D = aT + M is still below the critical threshold D0. This generates mutual repulsion that actively pushes metal atoms into the a-Si layer ― providing the initial driving force for interlayer mixing. When D reaches D0 during annealing, the system simultaneously crosses into the attrac- tive regime, triggering rapid condensation of Si into crystalline grains and aggregation of metal atoms, thereby completing layer exchange. The mechanism quantitatively explains the sharp onset temperature, the superiority of Ni/Pd over Al/Au, and the absence of layer exchange in noble-metal systems.