You are hacked in a way that there's a malware that I assumed you installed and you never noticed it so the hacker is free to check out on your computer.
I've always thought the JSON file is just as vulnerable as a private key?
There is no perfect system in the world so everyone can penetrate if there's an opening especially if the hacker is quite good and has been robbing as his job. This is why I'm always telling you guys that spend some money for your security through buying an hardware wallet.