A blunt and short reentry capsule tends to be dynamically unstable at transonic speeds, attributed primarily to the delay of base pressure. In the present study the flowfield around a capsule under forced pitching oscillation is numerically simulated, and the results are compared with that around the capsule at a fixed pitch angle. The two flowfields are found to be essentially the same except for a delay in the base pressure in the oscillating case. Detailed flow analysis reveals that impingement of reverse flow behind the capsule determines the base pressure distribution, and the behavior of the reverse flow is governed by the vortex structure behind the capsule. The vortex structure is composed of a ring vortex and a pair of longitudinal vortices, and the interaction between the longitudinal vortices and the flowfield near the neck point defines the flowfield behind the capsule. When the pitch angle is changed, the base pressure does not change until the disturbance of the longitudinal vortices caused by the pitching motion reaches the neck point. This time lag is the cause of the delay of the base pressure. This mechanism reasonably explains the characteristic features observed in the numerical simulation and supports several important features reported in previous experiments.