Well they don't know for suuuuure. We do know, but Kakashi can only suspect that Taro met with someone big like Orochimaru or Kabuto. Until he sees the Sound Ninja himself he won't be sure they're behind this but EVERYTHING points to Orochimaru really. He's the only one other than the Raikage who'd kidnap a child to get what he wants. Assholes, man.