There is precedent for why a contract year is so important.
It’s also worth noting that Mark Harmon is pushing 70 and could be ready to back off of the demands of shooting a lengthy TV series every year–particularly since he retains producing credits in the famous franchise.
If you just look back to a couple of years ago on the Eye Network, The Big Bang Theory’s Jim Parsons was the one to ultimately call the show’s final season after years of intense contract negotiations between the main series leads.
We see this all the time in sports, but even on television, as an actor wraps his or her contract, there’s always an opportunity to tackle or take on new things.