She was trying to get the secret service man from the back of the car up to the President.
After reading this I searched the web for jfk assassination video to see if I agreed with you. The part about Jackie climbing out to pick up bone/brain off the trunk just never made sense to me (even considering the circumstances). Anyway, I found some excellent vidcaps from the Zapruder film. The website was made an anti-Bush wacko but the caps are good so I'll just link to the videos themselves (they use the divx codec):
2. Close up Kennedy and Connally
The close up is very graphic (sickening, actually) but shows some interesting things. JFK was shot in the throat just as he emerged from behind the street sign. His arms come up to his throat in an automatic reaction. Connally must have been hit by the same shot because he jumps and turns to look back. You can see that he says "Ouch".
Jackie grabs on to JFK's left arm and appears to say something to Connally. She then realizes that JFK is hurt, too, and curves around in front of him to see what's wrong. That's when the horrible head shot impacts. Jackie's face was only a few inches away at the time. Startled, her right hand flies up to the back of JFK's neck as he slumps over. She then, without turning to look at the trunk, starts to climb over the seat, even pushing off of JFK's head with her left hand. When she's halfway over the seat the Secret Service agent runs up, stops her, and tells her to get back into the car as they speed off.
You know what I think? Do you care? Anyway, it looks to me like Jackie was, understandably, panicked from having her husband shot right in front of her. I think she was tring to climb out of the car to get away. She never looked back at the trunk until she was already climbing out. If she was going to retrieve a bone fragment, wouldn't she have had to see it first?