I doubt it is related to travel agents. If a ship fills up, they pull travel agent space. They don't let agents sit on it for very long if they think they can sell the cabins.
This itinerary is always pretty full as they only do a few Norwegian Fiord trips per year. I know when I went in 2018, I got a move over offer to September. They were doing the cruise I was on in June and September and that was it.