Developers like to develop, develop not only software, but also their careers. When an opportunity of becoming a line manager appears, it's usually considered as a promotion. That's a common misunderstanding because it's a role change, switching a career path. Scope of work for a manager is completely different, it's about people, not code, it also requires completely different skills. It's a great adventure, worth trying, but if you don't like, you can always go back to coding!