The natural instinct for most people at this stage of their careers is to first fall back upon those same skills that made them so successful as an individual contributor. But lo and behold, try hard as they may, the same skills suddenly seem completely inadequate while they grapple with their new role as a team manager. The parameter for measuring success for them has subtly changed from "doing a great job" to "getting a great job done". And that makes all the difference!
Majority of the managers who are promoted do not receive any formal training on their new role as managers. Little wonder then that they struggle in their roles! The skills needed to succeed as a people manager, are a whole new paradigm:

Transitioning from being an individual contributor to a people manager
Coaxing out great performances from team members
Communicating effectively with the team
The art of influencing others
Coaching for performance
Creating a positive team climate
Delegating for growth and success