Account Security is generally violated by one of two ways, first and most common is account sharing. Assuming you haven't done this, you've probably gotten a keylogger.

There is a 3rd, however I believe it's relatively rare and only affects those who have been nailed with a keylogger int he first place. But I believe that once these guys identify a username they run brute force attacks on the passwords... a guy in my guild had his account hacked/stripped 4 times with a complete reformat in between. He finally bought a new account, moved toons to it, and hasn't had a problem since.