You have probably been "subscribed" since you posted in it. It's the default option but can be changed in your User Control Panel >
User Options (Look for "Default Thread Subscription Mode")
In any thread you visit, if you look above the first post in the thread, or in the first post on a page of a long thread, there is a link that says "Thread Tools". It is a drop down menu. One of the options is "subscribe to this thread" if you aren't already, or if you are the option will read "Unsubscribe from this thread".
This is the Instruction - (broken link removed)
Note: You can also "Subscribe" to an entire forum.