A 67-year-old man undergoes transurethral resection of the prostate (TURP) for benign prostatic hyperplasia (BPH). Two days post-operatively, he develops confusion, nausea, headache, and visual disturbances. His serum sodium is found to be 122 mmol/L (ref 135-145). What is the most likely cause of his symptoms and hyponatraemia?