Cats try to hide when they are anxious or feel threatened. They especially like to hide in high places, which permit a clear view of their surroundings.
This is why it is especially important to provide indoor cats with hiding areas; these spots must permit the cat to feel safe from people, loud noises and other animals (see more information about perches).
If it is not safe for your cat to hide on top of the refrigerator, etc., then you will need to provide some other high location for your cat to hide in, such as a climbing tower or closet shelf.