This is just speculation on my part, but I think Brandi is behaving this way, because she got her character description from Bravo. This is who she is this season. Perhaps she gets bonus pay if she brings more attention to the show by fighting with her husband's wife. This thing with Adrienne and Paul will wrap up and next season we'll see a new theme. Perhaps, Adrienne and her new lover will take the lead then. Who knows.