User blog comment:AK151/So... Episode 9 (and some blind speculation)/@comment-18396318-20130913052709/@comment-16241789-20130913053104

Ruby was the one who came up with the plan. She was able to direct the group. In combat, she is shown to be tactically capable. Same reason Jaune was made leader. He took control of the team and directed them to victory (despite them probably being to handle the Deathstalker regardless).

Weiss in combat seems to have more of a support role: she froze the Nevermore in place, created what I like to call the attraction and repulsion "runes" that launched Ruby, and gave Ruby a path up the mountain.

It makes sense for Ruby to be leader as she was able to direct the team to victory. It just bothers me how she is behaving, considering she IS a leader; more is expected of her, and I think at some point she's going to have to understand this.