I work for an airline. Let me offer a much simpler explanation:
The seats are taken by other passengers.
Seats in the rear are popular because they are close to the lavatory. Depending on the airline, it may also be the first rows to be served in-flight meals.
One of the comments questions the pricing scheme. Airlines have plenty of data to determine their pricing strategies. We can aggregate months of data to see which seats are popular. That's thousands of flights.
One of the answers state this is done for Weight and Balance consideration. This is incorrect. A modern airplane, like an A320, has a wide enough envelope for us to not worrying about that when selling seats. And, if necessary, the operations department can offset it by loading cargo forward or backward. It may happen that we have to ask passengers to move seats, but it is exceptionally rare.
While it is true that certain seats may be blocked for operational reasons, I doubt any airline does this on a frequent basis. Seats are money. With profit margins being so low and the industry so competitive, blocking 10 seats on each of your flights is like shooting yourself in the foot.
Deadhead crew is a possibility. However, to have 10 deadhead crew occupying revenue seats, someone in the roster department has screwed up.
My guess, based on OP's screenshot, is that a rather large group is traveling together, and they have all selected the rear seats.