Unused account hacked?

I recieved the below email earlier today. However, I have not used or tried to access account XXXXX in over a year. What could have possibly happened? Could someone have just been playing on it for the past year and I never knew?
Thank you

Greetings,

We have determined that the World of Warcraft account XXXXX has been accessed/compromised by someone not authorized to do so by the World of Warcraft Terms of Use (http://www.worldofwarcraft.com/legal/termsofuse.html).

To protect your privacy and security, we have temporarily disabled this account. Any recurring subscriptions have been suspended to prevent further monetary charges. In order to regain access to the account, you must complete the steps below to secure the account and your computer.

Please keep this email for your reference until the account recovery process has been completed.

STEP 1: SECURE THE ACCOUNT, YOUR COMPUTER AND YOUR EMAIL ADDRESS
Account compromises most often occur when a player shares login information with an unauthorized third party or plays on a computer that has a virus, Trojan, or key-logger. We recommend you read and apply the following tips to protect yourself and the account.

- Unauthorized Account Access Policy: http://us.blizzard.com/support/article/20460
- World of Warcraft Account Security: http://us.blizzard.com/support/article/20572
- Computer Security: http://us.blizzard.com/support/article/21118
- Email Address Security: http://us.blizzard.com/support/article/28585

STEP 2: RECOVER THE ACCOUNT
We now provide a secure website for you to verify that you have taken the appropriate steps to secure the account, your computer, and your email address. Please go to this site and follow the instructions:
http://us.blizzard.com/support/article/securitywebform

STEP 3: VERIFY YOUR SUBMISSION WAS RECEIVED
We will contact you with further instructions once we have received and processed your submission. If you do not receive a reply within 48 hours of submitting this form, please resend it from the address listed above.

Please be aware that if unauthorized access to this account continues after the recovery process is complete, it may lead to further action against the account.

Regards,
Blizzard Entertainment
www.WorldofWarcraft.com