Guest post by "Tsuki_Ouji"
Actually, it makes complete sense. In a high-stress situation like that, many of your body's chemicals are getting pumped into you, double-time. The big ones are, of course, adrenaline and various hormones. Thus, the big damn kiss either before or after a climactic battle. Alternatively, they beat the BBEG and run for a shag as fast as their legs (or other analogous body parts) can carry them.