I don't know but I suspect what happened is someone was carrying their guns from one house to another all innocent like and they got stopped for some minor traffic infraction, like forgetting to use their turn signal, or speeding or some such. The cop checks out the car as is his duty and notices the guns lying in the back and that was all it took for it to become the policemen's ball of the night.
I'm pretty sure the law requires that guns carried in your car must be locked in the trunk. Didn't find anything that specifies that for Oregon, but I did find this on American Firearms Training:
Federal Law on Transportation of FirearmsSince SUV's don't have a trunk, the guns were probably lying in the back in plain view of anyone peeking in the windows.
A provision of federal law serves as a defense to state or local laws which would prohibit the passage of persons with firearms in interstate travel.
Notwithstanding any state or local law, a person shall be entitled to transport a firearm from any place where he may lawfully possess and transport such firearm to any other place where he may lawfully possess and transport such firearms if the firearm is unloaded and in the trunk. In vehicles without a trunk, the unloaded firearm shall be in a locked container other than the glove compartment or console. Necessary stops, e.g., gasoline and rest, seem permissible.