Your account or wallet information was compromised becauase the hacker needs to know your credential first.before he can transfer your coins.Are you sure that you computer doesnt have malwares?if no try on install malwarebytes it will help you to know what caused your problem if.the malware bytes dones detect any you probably got phished,that is why you need to use metacert extensions in your browsers so you wont be a victim.
I agree with you. In this case there are only two ways to get private key.
First, ask directly to the owner.
Second way is place a keyloger in the owner computer so they can tracking all activity on that.
So if you wasn't give your private key to someone then exactly your computer get some malwares. Reinstall your computer and next time be carefully when you want to download somethings.