As I understand it, there are programmes and servers that scan the internet looking for accounts to hack. They try for example to open our accounts using random passwords and they hit on some as someone somewhere will have that password. This happens more frequently to you if your password is not very cryptic. eg if your password is elvis or Michael Jackson, you'll be hacked most days!
So change the password and make sure you have a couple of numbers as well as letters but not too easy, so don't use name and birthyear for example.