Under certain conditions, the wake behind a square back Ahmed body is not symmetric with respect to its vertical mid-plane and is characterized by random intermittent switches from one quasi-steady asymmetric state to its mirror counterpart. The dynamics of the switching process can be described using POD analysis of the base pressure. The decomposition is applied to results from a numerical simulation (Podvin et al., J. Fluid Mech., 927, 2021) at Re=10 000 as well as experimental measurements obtained at Re=40 0000. It is shown that in both cases the switch is associated with a pressure increase over the full base, as well as strong variations on the external part of the recirculation zone. Analysis of the velocity field in the near-wake shows the predominance of a quasi-steady laterally asymmetric deviation between switches, vertically asymmetric patterns during the switch and oscillatory, vortex shedding motions. A POD-based linear stochastic estimation method developed in Podvin et al. (Exp. Fluids, 59, 3, 2018) is used to examine the relationship between pressure and velocity patterns. It is found that the base pressure can be well recovered from the velocity field in the near-wake, and that conversely some of the flow large scales can be correctly estimated from base pressure measurements. However the vortex shedding motions appear to be poorly correlated with the evolution of the base drag.