The best person to answer this question is a coroner. This is a medical and legal question that the Coroner's Office deals with every day.
In order to cause a lung to collapse, that usually need to involve either a puncturing injury from either blunt trauma to the chest wall (ie car accident, fall, stabbings, etc) or by intense pressures generated in the lung. Its been known that in some cases a cough, or sneeze can cause a pneumothorax (collapse lung or portion of a lung)
The coroner or detectives working on your brothers case may be able to piece together some info.
-Andy