What causes blood vessels to burst in lung?

Many things: Hemoptysis or coughing up blood from the lungs has several causes. Blood tinged sputum can happen from repeated, forceful coughing. Coughing up more than a tablespoon of blood usually comes from either a lung infection or cancer eroding into the lung blood vessels, a vascularized tumor in an airway or because of very high blood pressures in the lung (pulmonary hypertension) seen in mitral regurg.