Blood flow restored. When the blood flow is restored to area that was hurt or damaged the blood vessel is not able to maintain its elasticity so the blood products leak and the damage occurs since the blood is in a place it should not be. New research points to cardiac death being secondary to reperfusion injury and research is being focused on this issue for future learing.
Free radicals. It is due to free radicals that are formed during ischemia. Basically the sudden gush of oxygen itself is bad.