I wont actually call it hacking, in the sense that its mostly as a result of not having password encrypted private key (password for wallet that is being used to encrypt wallet) or your password can easily be broken by dictionary or bruteforce attack.
It is hacking but it happens when we do mistake of sharing our private key with other people we should not share our private key with those bad people who are in search of an opportunity to take away our money and give us lose, wallet gives us high security but we also will have to take care and not to share our private key with anyone.