Why does a person instinctively curl around his abdominal area in times of danger?

Because organs in the lower abdomen have the least inherent protection. Your brain is encased by your skull and has a cushion layer of fluid, both which serve to protect the gray matter with-in. Your heart and lungs are both protected by your ribcage. The organs in your lower abdomen are the most vulnerable, having only your muscular structure and skin for protection. if you are attacked. By curling around the area you are also protecting your face and throat, two more areas without a hard outer protective casing. When your opponent only has access to your arms and lower legs, he is not as likely to deliver a fatal blow.