A 58-year-old man from Delhi presents to the emergency department with severe headache, confusion, and nausea for 6 hours. His wife reports he has been drinking excessive water (3–4 litres over 2 hours) after a prolonged gym session. On examination: BP 128/82 mmHg, HR 88/min, RR 20/min, temperature 37.2°C. Serum sodium 118 mEq/L, serum osmolality 245 mOsm/kg, urine osmolality 95 mOsm/kg, urine sodium 32 mEq/L. What is the primary pathophysiological mechanism underlying his clinical presentation?