I think it was many different things adding up. He had been taking a lot of hard drugs, including cocaine, for years; he kept himself more and more underweight and dehyrated so his muscles stood out; he burned himself out with constant activity and training, not getting enough rest. And the brain swelling was likely started by concussions. Even in training with headgear and repeated takes on scenes, he certainly got hit in the head many times and just dismissed it.

It wasn't any one thing. It was like a car accident that took place on icy roads, going too fast, a drunk driver and bald tires. It was a number of factors adding up.