If you have notifications on when you reply to a thread it will add that thread to your notifications. By default as of a couple months back that is in for everyone. If you turn it off you will have to also delete the existing ones because turning off just stops new ones from being created, it won't remove exisiting. If it is turning itself on after you have turned it off let me know when it happens because I can tie it to a forum change such as new theme.