Why did your World of Warcraft account get hacked

Your account can get hacked in multiple ways. a small list would include

1. You or someone with access to your account bought gold from a gold farmer and gave them your account information.
2. Your computer or a computer you logged in with had a virus on it and it logged your password and email address.
3. If you have your password saved into a file on your computer a virus could have found it.
   

The best way to avoid being hacked is to keep your account information to yourself, do not share it with a friend.
You can also buy a Blizzard authenticator, it generates a random password ontop of your normal password. They can be downloaded onto a smart phone for free or you can buy one from Blizzard.