Dunno why, but this question reminds me of similar queries on ebay forums .. lol
Some of the answers to such a query on ebay forums would be:
"Maybe your account got hacked and someone subscribed on your behalf"
"It's a virus. Check whether your computer is infected."
"Did you get a phishing email in the last few days?"
Up top, click on UserCP, then Edit Options, then under Message & Notification > Default Thread Subscription Mode, change the default subscription notice to whatever you wish.