It's probably a natural fear that everyone has: I'm scared to death of being in spaces that give me only a few inches all around me, or getting stuck inside such a space, like when exploring caves. One of my nightmares is being buried alive (yes, I about flipped my shit when I saw Dean wake up inside his coffin in "Supernatural" ).
Elevators, closets, cars, etc? No problem at all. I get social agoraphobia more often than claustrophobia. Except for the tiny-tiny space version of claustrophobia, that gets me.