Actually, the mechanism of clotting upon death is the same in life. Now, since the heart stops pumping blood around the body upon death, the blood in a dead body essentially just sits in place in the vessels. In other words, the blood is in constant contact with the surface of the vessels. Such constant contact actually damages the blood. This happens throughout the body, and once clotting begins, it is virtually impossible to stop naturally.
Free radicals are molecules with unpaired electrons and are the result of normal metabolism.
However, when the body dies, the antioxidant systems die, too, and free radicals are left unchecked. By their chemical nature, they initiate destructive chain reactions which increase logarithmically. And as we saw already, damage to the blood would initiate the intrinsic pathway to begin blood clotting. Incidentally, blood can clot in the living body and cause a lot of problems. If atherosclerosis occurs to a vessel leading to the heart, it could starve heart tissue, causing it to die, and cause a heart attack as a result.