Exciton relaxation is widely assumed to be governed by electron–phonon scattering, with Coulomb interactions playing a largely static role in binding. Here, using first-principles real-time Bethe-Salpeter formalism combined with nonadiabatic molecular dynamics, we study the ultrafast relaxation of optically bright excitons into momentum-dark states in monolayer WSe2. We show that Coulomb-mediated couplings between excitonic configurations, strictly forbidden in a perfect lattice, are dynamically activated by phonon excitation. This activation enables long-range Coulomb interactions to dominate exciton relaxation, reshaping kinetic pathways and surpassing conventional electron–phonon scattering. These findings reveal a new paradigm in which Coulomb interactions actively govern nonequilibrium exciton kinetics, while phonons act as a dynamical switch that unlocks many-body relaxation channels.