What is the most common cause of pneumomediastinum ?

Microscopic alveolar rupture.

Clinical settings where this may occur:

• deliberate alteration in breathing pattern: marijuana smoking and cocaine inhalation, pulmonary function testing, mountain climbing, wind instrument playing, yelling, shouting, singing
• straining and other involuntary alterations in breathing pattern: childbirth, vomiting, seizures, coughing, sneezing, hi cupping, heavy lifting, athletic competition, straining at stool
• external pressure changes: scuba diving, air travel, mechanical ventilation, Heimlich maneuver
  

Other less common causes of pneumomediastinum include air escaping from the upper respiratory tract or GI tract.

Source

Mason: Murray & Nadel's Textbook of Respiratory Medicine, 4th ed.