Why are my two male cats suddenly hissing and chasing each other?

Cats will often do this if they have been separated for a period of time, especially if one cat looks or smells different - your female cat may have strange smells on her from her time outside. Most of the time, this reaction is mild and the cats will get along again after a day or two. If there is any serious fighting or if his aggressive behavior towards her does not seem to be improving, then you may need to separate them and gradually reintroduce them to each other - feel free to ask for additional help on this if needed :)

It's possible the stress of your being gone caused the behavior change. I would give Hippy a few days to rest, relax, and make sure she can get to her food, litter, places to hide, etc without being bothered and see if that helps. Sometimes, pain or underlying illness can also cause a cat to suddenly lash out and hiss or swat at other cats, so if the behavior continues or you see visible signs of illness, a vet visit to rule out health issues is best.

Cats exhibit this behavior sometimes when one leaves the house. Most of the time it’s more growling and hissing and running then anything else. Most cats will get over this quickly. Supervise them so that they don’t hurt each other and make sure one does not bully the other. If you have something they really like it such as canned cat food, you could try feeding both cats the food or treats close to each other and talking sweetly and petting both. I hope this helps and best of luck!

If only pets could talk! Cats can be very reactive and can also get frustrated easily. It’s possible they saw another cat out the window and are reacting. You may try to separate them for a few hours. Keep the environment calm and check on them frequently to make sure they are not agitated. You want want to close the blinds to block out any views of the other cat. If the behavior continues, have them examined by the veterinarian. I hope this helps and please feel free to post any additional questions.

If your cats have spent most of their time outdoors and now are spending time inside together then this change in environment could be causing tensions. If they are still outside all the time then it is possible that a new cat has come into the neighborhood and this has disrupted the social structure of the cats, or potentially one of the cats could be feeling unwell and therefore being more defensive, or they could feel they are having to compete for food/shelter. I would be sure both cats have equal access to food and water (they may need their own separate areas to eat/drink so they don't feel threatened) and litter boxes indoors. When indoors I would recommend the use of the Feliway Multicat diffuser https://www.amazon.com/CEVA-Animal-Health-D89410B-MultiCat/dp/B01AVHPFDS/ref=sr_1_4?ie=UTF8&qid=1503838561&sr=8-4&keywords=feliway+multicat, which is a pheremone product. I would consider an examination by your vet to look for underlying health problems as well.

This happens quite a bit actually. Could be territorial aggression - she left and now he is trying to tell her the house is his OR it could be that he feels threatened due to the way she smells when she returns. I suggest you start using some Feliway products. I would spray the carrier when you take the cats to the vet and use the diffusers (multicat) in the house. ( https://www.feliway.com/us ) ( https://www.feliway.com/us/Why-Use-Feliway/how-to-stop-cats-fighting )

Stress and illness can be two factors in sudden behavior changes in cats, so getting Kosinus checked out by your vet is a good first step. Cats that are in pain or sick may suddenly lash out at friends or other pets when they were normally friends. If Kosinus checks out healthy, then looking for stress factors is the next step. Do the two cats have to share one food or water bowl? One litter box? One bed or perch? These can be things that may become contested, and if one cat is pushing the other out may lead to aggressive behavior. Making sure there are multiple litter boxes, beds, and food and water dishes may help. Pheromone diffusers that can be plugged into walls of shared rooms or used as a collar can also be useful for decreasing stress and aggression among cats, and may help Kosinus calm down some and get along again.