Jackie went to the back of the limo to retrieve a piece of Kennedy’s brain.
I know it is not logical but in a situation like that illogical action often happens. She tried to pick it up and put it back into his head.
Is that what she said? My thoughts were that either she panicked when she saw his head blow apart and wanted out of the limo, or she was trying to get to the ss agent to let him know what happened. Unless Mrs. Kennedy said why she was trying to climb on the back of the limo, everything else is conjecture.