I also think it's because they feel better. But I wouldn't be so sure about criticizing them because of that. If you feel good, you learn better.
And also because if you are in an easier level, it allows you to pay attention to the little things in the language that you wouldn't perceive if you were just struggling to keep face, about to drown in the load of new language. The issue is a delicate one
erhaps they should really choose an easier level if they want to; obviously they feel the need to have the feeling they have accomplished something, which the new course does not give them. Eventually they will see that they have to leave that stage behind: people do not want to be babies all their lives, they want to grow. But they also need some security. And really, teaching is all about security and challenge, isn't it?