The instinct for self-preservation leads unattached cats to avoid humans, dogs, loud noises, and strange places.

SOURCE: cats.consultinghealth.com

Fear is an emotional state in the cat. It is normal for cats to experience fear when they come across a situation that they have not encountered before. Cats that are not socialized to people as kittens, for example, will feel fear when approached by people. Cats will also feel fear if in situations that they have learnt mean something bad. Cats most often respond to fear by running away, hiding or climbing up high. If cornered, however, cats that are fearful will often show aggression.

SOURCE: cats.org.uk