The spatiotemporal flow evolution in a scramjet engine combustor with two opposing cavities is computationally studied using the Reynolds-averaged Navier–Stokes approach. The steady-state flowfield throughout the flowpath is first examined, and then the fuel plumes from both the primary and secondary injector arrays are analyzed. Finally, an in-depth examination of the temporal evolution of flow near the cavity region is presented, featuring the interaction between shock waves and shear layers. Due to a backpressure rise in the downstream, oblique shocks formed ahead of the secondary injectors propagate upstream, further raising the pressure inside the cavities. The high pressure combined with the shock-induced thickening of the shear layers produces a narrow core flow that remains supersonic throughout the domain length. Entrainment of the fuel from the primary injectors into the cavities is sustained, holding stable flames in the combustor. Additionally, large adverse pressure gradients resulting from the shock-boundary layer interaction induce regions of slowed or even separated flow along the combustor walls. The secondary injectors supply fuel to these high-temperature, high-pressure regions, creating another flameholding mechanism. The combination of both flameholding mechanisms produces sustained combustion and stable flames in the combustor.