Just as a partial, very incomplete list, in no particular order...
Traumatic Hemorrhagic stroke. Traumatic Ischemic stroke. Spinal cord injury. Asphyxiation. R-on-T phenomenon caused by blunt trauma to the chest. Injuries to the liver, spleen, stomach, intestines, kidneys. Congenital defects aggravated by exertion. Asthma. Bronchospasm. Tracheal injury. Clavicle fractures resulting in injury to the subclavian artery or vein. Sternal fractures resulting in pericardial tamponade. Cardiac arrhythmias. Femoral or pelvic fractures with vascular injury. Dehydration and electrolyte imbalances caused by certain idiotic methods of "making weight" for class competitions.
I can go on, if you like.
None of these things are particularly likely, but they are all possible. Tournament rules have two basic functions: keep the match fair, and minimize the risk of serious injury. The risk cannot ever be totally eliminated.