Reintroduced Cat Is Hissing. If they show signs of aggression (e.g., growling, spitting, hissing, swatting), separate them again and restart the process of gradual reintroduction. A cat hiss often shows dominance over another adult cat.
If your cat is hissing at other cats, one of the most common reasons is because he is in pain. Agitation (unsettled, trembling) cat crying, growling, hissing. How do you introduce two cats when aggressive?