Cat Drama and Hissy Fits

I don't understand. Zulu will push the bedroom door open. When Bailey appears at it, because of course she's interested (and isn't Zulu? She did push the door open, after all...)- Zulu hisses and growls at her. What is up with that? Just get over it! Bailey doesn't make the first (vicious) move... but if Zulu starts hissing and growling, Bailey will do it back.

And Sydney is just trying to figure out how to get Zuzie's food...