You might choose to lead a group discussion, or you might find yourself drafted for the task.  Some of the most common reasons that you might be in that situation:

• It’s part of your job.  As a Defence Officer. you might be expected to lead group discussions regularly.
• You’ve been asked to.  Because of your reputation for objectivity or integrity, because of your position , or because of your skill at leading group discussions, you might be the obvious choice to lead a particular discussion.
• A discussion is necessary, and you’re the logical choice to lead it.  If you’re the chair of a task force to address , it’s likely that you’ll be expected to conduct that task force’s meetings, and to lead discussion of the issue.
• It was your idea in the first place.  The group discussion, or its purpose, was your idea, and the organization of the process falls to you.