Cats are territorial. Some can also be very aggressive. Cats will fight other cats to protect what they think is their territory. They also fight for resources like food and water. A male cat can also fight other males to exclusively mate with a female.
If you have a lot of cats, keep them from fighting by providing each their own food bowls. Keep everybody well-fed. Incentivize good behavior, punish bad ones (just use a water spritzer). Keep neighborhood cats from getting into your property using scents that repel cats like citrus, lemongrass, and lavender. There are also commercially available cat repellents.