Sheriff's Sgt. Michael Konecny says the van had just dropped off passengers Wednesday in Valencia when the driver suffered an unspecified medical condition and it accelerated suddenly, striking the guard.
The victim, who was not immediately identified, was pronounced dead at a hospital.
CBS released a statement offering condolences to the victim's family and friends.
The network says production on the show has been suspended while investigators try to determine what caused the accident.
"NCIS," a drama about Naval criminal investigators, premiered in 2003.