Monolayer (1L) transition metal dichalcogenides (TMDs) provide a unique opportunity to control the valley degree of freedom of optically excited charge carriers due to the spin-valley locking effect. However, a unified picture of competing intervalley coupling processes in 1L-TMDs is lacking. Here, we apply broadband helicity-resolved transient absorption to explore exciton valley polarization dynamics in 1L-WSe(2). By combining experimental results with microscopic simulations, we dissect individual intervalley coupling mechanisms and reveal the crucial role of phonon-assisted scattering in the fast decay of the A exciton circular dichroism and the formation of the dichroism of opposite polarity for the B exciton. We further provide a consistent description of the valley depolarization driven by an intervalley-exchange-activating momentum-dark Dexter process and indicate the presence of efficient single electron spin-flip mechanisms. Our study advances understanding of exciton dynamics in TMDs.