How come I get subscribed automatically?

This happened to me once as well, a thread appeared in my subscribed threads list out of nowhere. After removing it, I realized I didn't even know how to subscribe to threads. I noticed the box you can tick when posting, but is there supposed to be way to subscribe to a thread without posting in it? Never mind, there's the button. I kept looking lower on the Thread Tools list to where the moderation options are.

I eventually figured I had ticked the "subscribe to thread" box accidentally or my keyboard did it for me through some miracle (tab, or some other freaky key combinations)

Jam it back in, in the dark.