Anybody know how to prevent this BS?

February 2nd 1462, this happens every single time; the Ottomans declare war, and the Austrians annex my Hungarian vassal. Like wtf? Why does this happen?

The Ottoman War is the lesser issue, cause they're actually pretty weak right now, and I can easily beat them, but what the hell is up with Austria? Is the game just coded to always give a Hungarian fiefdom to Austria, or something?