It can be distressing when cats who once got along suddenly start fighting. Several factors can contribute to this change in behavior, including unmet needs, underlying health issues, frustration, fear, and territorial disputes.

Unmet Needs
Cats have natural instincts that need to be fulfilled. Sometimes, what might appear as aggression is actually a playful attempt to satisfy their hunting drive. For instance, a cat might engage in play-like attacks, such as batting at another cat’s tail. Ensure your cats have plenty of opportunities for physical and mental stimulation.
Underlying Health Issues
A cat's emotional state is often linked to their physical well-being. Chronic conditions and idiopathic diseases can lead to mood instability and behavioral changes. If your cats are suddenly fighting, it's crucial to consult with a veterinarian to rule out any medical causes.
Frustration
Just like humans, cats can become frustrated. When cats experience frustration due to changes in their environment or other factors, they may displace this negative emotion onto other cats. Identifying and addressing the cause of their frustration is vital.
Anxiety and Fear
Changes in the environment, such as adding a new pet or furniture, can trigger anxiety and fear in cats. This can manifest as aggressive behavior. Providing a calm, safe, and predictable environment is essential to keep your cats feeling secure.
Territorial Disputes
Cats are naturally territorial, and each cat has its perception of its territory. If a cat feels that its territory is being threatened, it may resort to aggression to defend its space. Ensure that each cat has their own designated areas with food, water, beds, and litter boxes.
Resource Competition
Competition over resources such as food, water, toys, and even attention from their human can lead to fights. Ensuring that your cats have access to an adequate supply of resources can prevent this issue. Consider having multiple feeding stations, water bowls, and toy bins spread throughout the house.
Personality Clashes
Sometimes, cats simply have incompatible personalities. Some cats might be naturally assertive and likely to provoke other cats, while others may be more timid and easily irritated. While you cannot change their core personalities, you can mitigate conflict by providing individual space and opportunities for enrichment.
By understanding the underlying causes of sudden fighting, you can take proactive steps to address the issue, creating a more harmonious environment for your feline friends.