Pre-ordering something would usually cause a $0.00 transaction to confirm the card details are valid. It would be a ‘pre-auth’ transaction where the merchant reserves an amount on the card for payment at a later date, when they ship the item. If a fraudster makes a pre-order they xan validate that the card details are valid, then cancel the order, usually leaving the victim none-the-wiser. In your case, the bank noticed the transaction and notified you, but that seems to be rare. Once the fraudster knows the details are valid, they can sell them on.

It’s just a theory, and unless your bank and Blizzard work together to track the transaction, why it happened, and who instigated it, its going to be difficult to get to the bottom of it.