<p>The present study investigates the generation of negative peak pressures on a two-dimensional square cylinder within a separated flow. Large Eddy Simulation is conducted to obtain the flow field with high-resolution at a Reynolds number of 22,000. The trailing edge of the square cylinder is recognized as the area most susceptible to negative peak pressures with an abrupt and large peak in the upcrossing frequency distribution. The pressure wavelet decomposition and velocity Dynamic Mode Decomposition are used to develop a pair of synchronized quasi-periods of pressures and velocity fields. Based on the synchronized quasi-periods, a periodic merging phenomenon between the large-scale vortex at the lateral surface and the Karman vortex immediately downstream of the cylinder is detected, coinciding with negative peak pressures. This merging process intensifies the recirculating flow around the trailing corner, leading to its subsequent separation, and culminating in the formation of a small vortical structure just upstream of the trailing edge. The large-scale vortex interaction is then recognized as a fundamental prerequisite for the generation of negative peak pressures through a synchronized observation of the real-time wind pressures and velocity fields. Furthermore, the impact of the small-scale vortex dynamics on the generation of negative peak pressures is discussed, providing complementary insights into the overall phenomenon.</p>