He found that they died, not just from drinking too much water, but because their kidneys stopped excreting water as a response to skeletal muscle injury.
When runners "hit the wall" they force their muscles to continue to exercise even after they've run out of glycogen, or fuel, he told Reuters Health in an interview. This triggers a stress hormone in the brain to tell the kidneys to halt water excretion, in an effort to maximize blood volume.
When this happens, even a relatively small amount of fluid can cause the brain to swell. If a person continues to consume fluids, the body reacts to the inflammation by continuing to protect blood volume, making the brain swell even more.