1. Someone starts a thread on a topic.
2. Lots and lots of people post on that thread, so that it becomes unmanageable to maintain the discussion just in that thread.
When those things happen, we make a sub-forum to contain the large volume of chatter on that subject.
A good example of this - in 2009, we had a lot of threads started on the topic of cooking healthy foods on a budget...so many that we thought it'd be easier to manage them if they were all in one place. And "Shoestring Meals" was born!
So start a thread for your discussion and see where it goes!