As a matter of policy, NW does not overbook F class, but generally overbooks C class on international flights by 1 or 2 seats.

Given that the aircraft was not downgraded, I can only assume that Federal Air Marshalls booked the flight, their reservations are generally made within 24 hours of the flight and are always F class, unless they are not on duty. It makes no difference if F is already booked full or a certain aisle seat is already taken, if the FAMs want it, they get it and the paying customer unfortunately pays the price and the agents cannot disclose why a customer was downgraded or moved from their pre-reserved seat.