I am hazarding a guess that it has to do with latest activity. In my own list, it seems to go by most recently begun. The older ones seem to gravitate toward the bottom even when they are the most active....but maybe the moderator can help us understand.
I just would like, personally, for the ones I once "read"ONLY--- to just disappear...and just leave the threads I post to....
Good idea to ask.
There is a topic list order when you open the name of a forum room, and there is also a topic list order in "Hot Topics" and "My Topics". I hope I am able to keep them all straight in this brief explanation.
When you open a forum room by selecting the room name, you get a list of all of the active threads started in that room. Some threads are informational and are permanently pinned at the top of the list. (See "Regional Interest" room for an example). Otherwise, it's by chronological order of all of the threads started in that room.
In the "Hot Topics" tab, it is chronologically ordered by activity, if that makes sense. So, the most recent replies in an active thread cause the thread to appear at the top.
In the "My Topics", I am not quite sure. It appears to be chronological by the order of the threads you have visited, but I could be wrong, as sometimes I bounce around from thread to thread, and I can't even keep track of what order I posted or read something in a given thread.