S2 is due to:

The Second Heart Sound Is Associated with Closure of the Aortic and Pulmonary Valves.  The second heart sound results from sudden closure of the semilunar valves (i.e., the aortic and pulmonary valves) at the end of systole.  When  the  semilunar valves close, they bulge backward toward the ventricles and their elastic stretch recoils the blood back into the arteries, which causes a short period of  reverberation  of  blood  back  and  forth  between  the walls of the arteries and the semilunar valves, as well as between these valves and the ventricular walls. The vibrations occurring in the arterial walls are then transmitted mainly along the  arteries. When the vibrations of the vessels or ventricles come into contact with a “sounding board,” such as the chest wall, they create sound that can be heard.   Reference: GUYTON AND HALL TEXTBOOK OF MEDICAL PHYSIOLOGY, THIRTEENTH EDITION(INTERNATIONAL EDITION ) page no 282