Possibly a password theft if your password is too easy to guess (several words you always use) but in order to steal your coins, the hacker must also have access to your private key. In fact there is no 100% sure way to make sure that no one else have your private key unless it is generated by you manually and kept offline secretly
The entropy in core client should be strong enough to avoid key collision, but the key should be generated offline and kept offline most of the time
The passwords were about 30 simbols and didn't contain whole words
I've never entered the passwords for many mounthes
It is possible that your private key is already stolen before you set the password. For example, some Trojan has been installed before you install the core client, and they constantly searching for the wallet.dat file on your c drive. Once found, they immediately send the wallet to the hacker, and that is before you even set the password