What triggers a default thread subscription?

Sometimes I see an interesting thread and explicitly subscribe to it. When I post to a thread, it appears that the Board automatically subscribes me to it.

However, my User CP is showing me subscribed threads that I have viewed but neither posted to nor explicitly subscribed to. Why is that?

Please see User CP/Edit Options.

Then look under “Messaging and Notifications” for DEFAULT THREAD SUBSCRIPTION MODE.

“When you post a new thread, or reply to a topic, you can choose to automatically add that thread to your list of subscribed threads, with the option to receive email notification of new replies to that thread.”

Look at the drop-down menu for the selection that you like. If you click “do not subscribe” only the threads you directly select will be on your subscription list; otherwise all threads you post to will be on the list.