As previous posters have already indicated, conspiracy theories concerning the John F. Kennedy assassination were widespread before the Zapruder film came to be televised regularly.
One reason the film is of such interest to some conspiracy theorists is that it clearly shows the President’s head snapping back as it is hit. This gives the impression that shots must have been coming from in front him, while Oswald, of course, was behind him.
CBS reporter Dan Rather was the first journalist to see the Zapruder film. He reported, falsely, that Kennedy’s head could be seen to lurch forward as it was hit.
I have read that Life Magazine edited the issue in which it first printed stills from the Zapruder film. In the first copies to be distributed, it is said that that Kennedy’s head appears to toss back. In the majority of copies, the selection of stills reprinted is different, and the motion of Kennedy’s head is not clear. While some conspiracy enthusiasts have claimed that Life was acting as part of a cover-up, the magazine soon came to support the belief that a conspiracy had been involved.
Some conspiracy writers have made this movement a cornerstone of their arguments. For instance, David S. Lifton, author of The Best Evidence, writes about the motion of Kennedy’s head continually in his book and insists that this can only mean that shots came from in front of him. While he offers no actual analysis or proof for this conclusion, he refers constantly to his background as a “physicist”.
Lifton, if I understand correctly, had a job buying and installing computer systems at the time he wrote his book. If he had any particular background in ballistics, he does not mention it. A physicist of rather greater distinction, Nobel Prize winner Luis Alvarez, undertook a detailed study of the Zapruder film and concluded that Kennedy’s head was moving precisely as would be expected if all of the shots which hit him came from the book depository.
The movement of Kennedy’s head was also a key element in Garrison’s arguments, and is much emphasized in the highly entertaining (but highly misleading) film JFK.