No doubt because you reused the non-trivial password and someone read it out of a stolen database.
Password strength is a red herring. Password reuse is a far bigger problem then weak passwords.
Password strength is a red herring. Password reuse is a far bigger problem then weak passwords.
Even if the password was unique to iTunes, a key logger on your computer could have intercepted it.
Does Apple always use HTTPS? Otherwise it might have been intercepted when you were on an insecure wifi network.