The definition of claustrophobia is:
An uncomfortable feeling caused by being in a situation that limits or restricts you; the fear of being enclosed in a tight space.
It is considered an anxiety disorder. There are a couple of causes:
Genetics- basically you are born with it and it takes a particular event to bring it to light.
Classic conditioning- events of confined areas eventually develop into a “disorder”. A child may develop the condition at a young age. Being left alone in a vehicle, accidentally being in an elevator alone or an older sibling locking them in a closet.
Amygdala- the fight or flight response. It is a very small part of the brain. It is thought that panic disorders and phobias are caused by this particular part of the brain.