Once it comes in a career the time to lead the teqam, so many things change.
You leave the “let me finish the task successfully” zone and enter the zone of “I need to drive, inspire and teach others how to finish successfully the task they are working on both individually and as a team”.
From the zone of technical competencies you enter the zone of management skills and leadership, with the team in front of you being your major task.
It often happens that top experts get a chance to run the team and they fail to do it successfully. The most frequent reason for this is that we tend overlook the fact that expertise in technical part of the job is one thing, while management and leadership are something completely different.
Help your key people through strong mentoring and coaching to run their teams where needed and in the right manner.