What could cause a lung to collapse?

Injury or luck: Ok, not really luck. Some people are prone to "spontaneous pneumothorax" where the lung collapses on its own. Other reasons include injury, infection. Treatment may include drainage with a tube or even surgery. Treatment options depend on cause, patient symptoms and conditions. More information necessary to properly answer this question.

Hole in lung: Non-trauma, spontaneous pneumothorax occur as a result of an abnormality in the lung tissue. Risk factors for primary and secondary spontaneous pneumothorax (sp) include: smoking (including marijuana), tall thin stature in otherwise healthy, connective tissue disorders, pregnancy, familial history. Sps are associated with 20-40% recurrence rate (depending type).