Sounds like one of the many zombie reservations that were discussed in the past, ie. a cancelled reservation that comes back to life. In your case the system, defaulted to a FNA as that's always the first option that Marriott defaults to when you make a new reservation. You should definitely call ASAP and re-cancel the reservation.

No-show charge for cancelled (and confirmed as cancelled) reservation. - FlyerTalk Forums

Still Charged For Cancelled Booking - Grounds For Refund & Compo? - FlyerTalk Forums

Zombie Reservation-Keeps Coming Back from the Dead - FlyerTalk Forums

It's nothing nefarious. Just Marriott IT being Marriott IT.