I'm a firm believer in not over training. I strongly believe in load management.Read the latest book about him.....Bruce Lee: A Life.
Excellent work. The latest theory based on all the evidence now just coming out is that he died from a heat stroke from over doing his training.
There are two major causes of injuries.
1) RSI --- repetitive stress injuries --- Tiger Woods' back and knee injuries are an example
2) Contact/collision --- getting wack on the head with a hockey stick or elbow