Bronn still wants his castle
He was promised Highgarden by the Lannister brothers and he'll be damned sure that they pay up.
Kings Landing devastated? Cersei nowhere to be found? Bronn hears that the Dragon Queen is around there and that means the little snot Tyrion is not too far away. I guess if she's the new boss then she can offer up Highgarden. If not, then it will be an arrow through the heart for her. No messing about. ;)