Sometimes it may be an act, consciously chosen for strategic reasons. But often it's a conditioned response, and the conditioning starts early.

Research studies have shown that, even in elementary school, girls are less likely to answer questions in class than boys, even though they are just as likely to know the answer. (Even more upsetting, teachers tend to call on boys before girls.)

I think it's cultural expectations. Males are expected to lead in most cultures, and females to follow. Which IMO is kind of backwards. Speaking as a former psychologist, I think most men are too emotional to be trusted with positions of great responsibility.