Abstract Back‐propagating earthquake rupture (BPR), where slip migrates opposite to the main rupture direction, is observed in several earthquakes but its origin remains poorly understood, particularly the role of fluids. We use dynamic simulations of frictional sliding coupled with fluids to explore the mechanisms of BPR. Our simulations show that BPR occurrence and rupture mode are controlled by the ratio of initial shear stress to effective normal stress τ0/σeff $left({tau }{0}/{sigma }{text{eff} }right)$: low τ0/σeff ${tau }{0}/{sigma }{text{eff} }$ produces pulse‐like rupture with BPR, whereas high τ0/σeff ${tau }{0}/{sigma }{text{eff} }$ promotes crack‐like rupture without BPR. Fluids modulate this ratio by reducing τ0/σeff ${tau }{0}/{sigma }{text{eff} }$. Localized patches of differential pore pressure act as asperities or barriers, inducing slip rate and shear stress perturbations that generate a secondary backward‐propagating front. BPR emerges from incomplete stress release, temporal healing and stress perturbations. These findings highlight the role of fluids in rupture dynamics and provide an energy framework for BPR in earthquakes.