Cats are by nature very territorial. This is because they have developed from a species that needed to maintain a territory in order to catch enough food to survive. Even though we feed our modern day cats, they still see it as a very important part of life to maintain a territory. Where there are a lot of cats in an area, territory sizes can be quite small, and some more nervous cats might find it difficult to maintain a territory at all. This is a common cause of anxiety in cats. Cats living in social groups will share areas of territory. Cats that are not part of a social group coming into a territory (even if they live in the same house) will be perceived as a threat by the resident cat.

SOURCE: cats.org.uk