How many devices do you use in browsing the forum? or how many browser's are you logged in? probably still watched in one of the devices hence when you unwatch from one place it still remains the same in the other places you're logged in untill you log out of those places then unwatch it will probably unwatch.

And additional if you recently made a post on that thread and in the "Additional options" in the down left corner of your message box/input box click on it and check if your "watch this topic" is marked blue, uncheck the box, because it automatically watches a thread for you without your noticed since it's on.