How did Diamond Tiara get her cutie mark?

Here's something that's been bugging me. How did Diamond Tiara get her cutie mark?

Here's what got me to ask that:

• In "Crusaders of the Lost Mark", it's shown that her talent is leadership.
• She had her cutecenera in "Call of the Cutie", so she must have gotten it recently then.
• If the "proper" way to leadership is guiding people rather than dominating them, and that she was bossed around by her mother her whole life, than what would have warranted her to earn her mark.