Excessive bleeding during hemothorax is caused usually by-
Massive Hemothorax:

Collection of more than 1500 ml of blood in the pleural cavity is called massive hemothorax.
Massive hemothorax is usually the result of major pulmonary vascular injuries or major arterial wounds, while minor lung injuries cause small hemothorax.

Also know 

Most common cause of hemothorax is due to injuries of Internal mammary Artery.
Vena cava
Internal mammary artery
Heart
Major artery