Wars for duchies

[americanu197]

So i find a claimant for a duchy and i make war but after i win the new duchy is independent not my vassal despite the fact that im a king and on the display it says my character becomes new liege
Why is that

 

[Pallen]

If the Duchy you are pressing a claim on, on behalf of another person is not in your de jure kingdom, then that duchy will become independant.