EDIT: There's no "rack" mentioned in the source. I assumed that. A loose loaded gun makes even less sense, tho.
Loaded while in a vehicle-mounted rack. Not just illegal, but also violates some fairly fundamental gun safety principles.
Heck, just "treat every gun a loaded" and "never point a gun at anything you don't want shot" means we should probably do away with most vehicle-mounted racks to being with. I suppose inside the metal toolbox in the truck bed would be okay; at least then the dog wouldn't fire it.