Most likely your fault.
Even if someone found out your login from the leak in june and used this knowledge to find out your password from some another service it's your fault you didn't create new account.
MtGox safety is limited to obvious bruteforce attempts, user database protection, their wallet protection and small stuff to keep the data you entrusted them from getting out. They reacted fast during the last crysis and limited the damage.
Everything else is entirely YOUR fault.