Your cells are only but so strong and with over-exertion and high water-loss the cells become leaky and the ion Potassium escapes in higher concentrations than is needed for muscle contraction. With too much potassium, your muscles will seize up. There may be other complications like liver and kidney damage as well, but those are usually only in the most severe cases.
I didn't mean to imply that there isn't such a disease, just that it was very convenient for so many people to cry rabdo after getting a good beating. A lot of those people wound up DORing or getting attrited anyway for failure to train.