A 72-year-old woman with a history of hypertension (on thiazide diuretics) and recurrent urinary tract infections is brought to the hospital with severe nausea, vomiting, and a fall at home. On examination, she is orthostatic (BP supine 110/70, standing 92/58 mmHg), mucous membranes are dry, and skin turgor is reduced. Laboratory findings: serum sodium 124 mEq/L, serum osmolality 262 mOsm/kg, urine osmolality 320 mOsm/kg, urine sodium 18 mEq/L, blood urea nitrogen 48 mg/dL, creatinine 1.8 mg/dL (baseline 0.9 mg/dL). What is the primary mechanism underlying this patient's hyponatremia?