This paper presents an in-depth study of the impact of a surge on a vertical wall using incompressible and compressible RANS model simulations of a classical dam break experiment over a dry bed. The model allows access to the detailed flow structure and pressure field at any instant, which provides valuable complementary information to measurements. Our study focuses on the second force peak, which is often the largest one and for which the literature does not really provide a clear explanation. Before and after this peak, the pressure on the wall is governed by the flow kinematics in the area. Before the peak, an overpressure appears at the root of the reflected jet, corresponding to the violent interaction between the incoming surge and the run-down flow. At the peak instant, the situation suddenly changes, due to the collapse of the reflected jet onto the incoming flow, trapping an air cavity. As in the classical case of direct wave impact on a wall with a trapped air pocket, this process generates an additional strong uniform pressure field in the air cavity, which propagates to the water and nearby boundaries due to the water confinement effect. This compressible effect, which varies depending on the capacity of air to escape the cavity, explains the formation of the second force peak. Finally, the 3D incompressible model provides a much more reliable estimate of the second force peak than the 2D incompressible model. This is likely due to the air escape phenomenon, which occurs when the experimental initial conditions are not perfectly 2D. Although it is unlikely that the 3D simulation perfectly reproduces the experimental flow, nevertheless, with more or less comparable air escape, the computation results appear consistent.