This is a prime example of Newtons third law of motion, for every action there is an equal and opposite reaction. The Police Officer unable to stop himself on his own accord is forced to transfer his kinetic energy to the young woman. While a woman's body, particularly the pelvic region, is able to handle small repeated transfers of kinetic energy, the whole of the body is unable to handle such vast amount of kinetic energy in a single burst. Thusly the woman looses balance.