Happens for a reason. Adolescents & young adults have "spontaneous" pneumothorax more frequently than other folks. The lung tissue usually has a minor congenital defect called a "bleb, " that bursts leading to a leak of air outside the lung - pneumothorax ensues. These folks are usually tall & lean. Of course, trauma can lead to pneumothorax, as can certain illnesses, particularly, marfan's syndrome.