Why did Jackie Kennedy climb out onto the back of the car after President Kennedy was shot?

Back in the day (well, the 90s), a teacher of mine told me it was to retrieve a part of her husband's brain, and at least one site agrees with that:

http://jfkhistory.com/jackie.html

The Secret Service agent in question, Clint Hill, who had Mrs. Kennedy as his protectee, has stated in interviews that Mrs. Kennedy was trying to retrieve bone and brain matter from President Kennedy's head wound, which had been blown onto the trunk of the limo by the gunshot. Photos taken at the time caused some commentators to speculate that Mrs. Kennedy was trying to help Agent Hill up onto the back of the car.

http://detroit.cbslocal.com/2013/11/20/former-secret-service-agent-gives-first-hand-account-of-jfk-assassination/