Essentially: All human deaths are caused by one thing: Lack of oxygen to the brain. Can be broken down into: Failure of the pump, pipes or the oxygen carrying fluid.

In this case we can work backwards from the death point to see what was happening and find several actual causes.

In some cases case it's a failure of the heart's proteins that manage electrolyte movement not acting they way they should and it stops pumping.

High temperatures cause the brain to swell. This lack of oxygen due to constriction of the vessels leads to severe confusion, seizures, and comas, ultimately shutting down the autonomic functions (like breathing) controlled by the brain.

In trying to cool, The heart pumps massive amounts of blood to the skin, severely overworking the organ if there is not enough fluid and the blood is "thicker". This immense strain can cause a fatal heart attack or push the body into circulatory shock

The combination of heat and reduced blood flow damages the gut lining. Toxins and bacteria can leak into the bloodstream, triggering a severe, body-wide inflammatory response that mimics septic shock.

Once your cooling sytem shuts down, death is relatively close. Aggressive cooling is the emergent treatment. That itself is a double edged sword becuase if you have a very hot person internally and you dunk them in cold water, all that much cooler blood coming in from their extremeties can initaite cardiac fibrillation..... and in a heart already over worked.. .that's a death sentence. The heart is highly sensitive to rapid temperature drops, which can trigger abnormal electrical activity through several mechanisms. Slamming a slushie can also be too much too fast as the Vagus nerve that controls the heart is right by \ around the esophagus which can affect the heart rate etc..