Poor training, poor dieting and probably poor medical attention. Fighters have this ignorant mentality that they must train hard everyday to make gains. I seriously doubt the validity of this ideology.
I think fighters should seek and invest in working with qualified sports medicine and physiology experts. I have this hypothesis that a fighter should check his inflammation levels when he feels at his best when he is out of camp.
When he starts camp, after every hard training session, the fighter should check his inflammation levels again. If they are way above the baseline, the fighter should do lighter exercises until the inflammation comes down and/or take non-USADA banned substances that reduce inflammation.
Once the inflammation has come down, the fighter should do another hard training session. Unreasonable stress and inflammation are beginning of the end. I could be wrong though.