Microtrauma to the muscles are created faster than the body can heal them.
Amino acids are used up faster than they are supplied in the diet. This is sometimes called "protein deficiency".
The body becomes calorie-deficient and the rate of break down of muscle tissue increases.
Levels of cortisol (the "stress" hormone) are elevated for long periods of time.
The body spends more time in a catabolic state than an anabolic state (perhaps as a result of elevated cortisol levels).
Excessive strain to the nervous system during training.
Other symptoms
Overtraining may be accompanied by one or more concomitant symptoms:
Persistent muscle soreness
Persistent fatigue
Elevated resting heart rate
Reduced heart rate variability
Increased susceptibility to infections
Increased incidence of injuries
Irritability
Depression
Mental breakdown