Accounts hacking
The problem right now is most likely a third party site got hacked.
Which gave hackers access to emails and passwords. Now the site’s password might not have been the same as the game’s password. But it might have been the same as the email password.
And any who bought the digital version of the game likely has their serial number in their email still. Given anyone who gains access to their email the ability to change the password (and then delete the evidence).
A lot of the recent hacks have had email compromises alongside of them. So it’s likely not ANet’s security that’s to blame. It’s the email providers and the fan site.
Moral of the story: Have a separate email you use for registering at fan sites and don’t share passwords with anything you don’t want to lose. And most certainly keep email passwords unique. Because email access is usually the only requirement for password changes on various sites.