Principal Skinner of Springfield Elementary School apparently suffered from posttraumatic stress disorder. What was one of the supposed causes?

Principal Skinner's constant, desperate, and usually ineffective attempts at maintaining discipline are an effort to receive good reviews in the frequent inspections of his very strict boss, Superintendent Chalmers—who makes no effort to hide his disapproval of Skinner. These inspections usually turn awry due to Bart Simpson's elaborate pranks. Over the years of pranks and inspections, though, Skinner has developed a love-hate relationship with Bart. One of Skinner's defining traits is that he had served as a sergeant in the United States Army during the Vietnam War, where he was captured by the Viet Cong at the Battle of Khe Sanh and spent 18 months as a prisoner of war. Seeing his entire platoon devoured by an elephant was one of the many things that led to the development of his posttraumatic stress disorder.