No one really knows for sure, though there is plenty of speculation about possible reasons or motives - some of which delve into the conspiracy realm of course.

What we do know is that Rather was one of the only members of the press who was allowed to view the Zapruder film the day after the assassination. We also know that the general public was not allowed to see the footage until it finally aired on March 6, 1975, over a decade after the shooting took place (though it was shown and analyzed privately at the trial of Clay Shaw in 1969, five years after the assassination).

Rather's claim was that when the bullet impacted JFK's skull, his head "went forward with considerable violence", and there is little doubt his report on what he saw in the footage in regards to JFK's head movement was totally inaccurate. Rather's claim that the footage showed that the President's head moved forward violently when he was shot in the head, because that was all the public had to go on in reference to the Zapruder film, certainly appeared at the time to support the official narrative that the headshot must have come from behind (where the schoolbook depository was located). From the conspiratorial standpoint, that has since made some feel that Rather was perhaps deliberately told to mislead/misreport the truth of what he saw so as not to feed the "multiple shooters" theory.

Perhaps incompetence is the real answer, with Rather simply doing a very poor job of remembering and reporting to the public what he saw after he was allowed to view the film. Eye-witness testimony is notoriously unreliable after all, and although Rather did not witness the actual shooting, he could be considered similar to an "eyewitness" when he was allowed to view the Zapruder footage the next day. He was not shown the raw footage more than a couple times, and it was shown to him at full speed. Today of course, we can watch the Zapruder film both in slow motion and motion-stabilized, making it far easier to analyze and interpret.

Dan Rather has commented about this blatant error in reporting he made of the Kennedy assassination before, and in his 1977 autobiography The Camera Never Blinks, Rather said the following in response to why he claimed JFK's head moved forward - "At the risk of sounding too defensive, I challenge anyone to watch for the first time a twenty-two second film of devastating impact ... then describe what they had seen in its entirety, without notes."