The hyponatremia or 'overhydration' associated with prolonged exercise arises primarily When blood sodium concentration falls to abnormally low levels, a rapid and dangerous swelling of the brain occurs, that can result in seizures, coma, and death. Sourced from: https://sportmedbc.com/article/hyponatremia-or-overhydration%E2%80%94who-risk

The intake of too much water leads to a condition called hyponatremia/ overhydration that results from the low concentration of sodium ions in the body.

The World Health Organization (WHO) has documented this condition in mountain climbers who have used melted snow to prepare their beverages, without supplementation with the necessary ions or electrolytes. Sourced from: https://www.beverageinstitute.org/article/hydration-for-athletes-special-considerations/

That leads to a number of problems, which can result first in poor performance, but also could lead to DNF and possibly death if not addressed. Sourced from: http://www.succeedscaps.com/articles/over_hydration/ The extracellular fluid around cells has to be more concentrated that the fluid inside the cell to prevent intake of excess water in to cells

If it is actually cold, and other signs of over-hydration are not present, shivering may just be due to poor thermoregulation. [youtube video=https://youtu.be/t1nwSuWr_q8 ] Sourced from: http://www.succeedscaps.com/articles/over_hydration/