The Bengals have historically taken a somewhat conservative approach to the draft. They generally hold most of their picks and take the players that come to them. This has hurt them at times when players they want go just before their pick, but they preferred to make all of their selections rather than use those picks to move up.
Trading back, is nothing new. Just last season they traded back in first round to add Cordy Glenn and traded back in the second round, knowing they could improve their third round pick while still acquiring Jessie Bates III.