Sorry to hear this. Change your buddy-auth page password ASAP. Email support staff from the email associated on the account and ask for the key to be changed.
You're fine, buddy team will fix your issue with a new key.
How could it happen? Great question. I would venture to guess, you've been on some other sites for botting WoW and maybe downloading like a "fishing bot" or "passive combat routines" or even more likely *lol@chatrestriction*.... lets say, 'illegal' version of x bot". Maybe not. But OC and CD are known to have bad files posted all the time. Great place to infect pc's.
It's also possible that someone is attempting to *lol@chatrestrictions*... lets say break the hashed keys and look for patterns in their cryptography. I find this unlikely but possible as there are so many examples of peoples keys floating around out there.