A 55-year-old man on ICU is receiving therapeutic anticoagulation with IV unfractionated heparin for a large proximal DVT. On day 5, he develops acute bilateral adrenal haemorrhage (confirmed on CT) presenting with haemodynamic collapse, hyperkalaemia, and hyponatraemia. His platelet count has dropped from 220 to 60 × 10⁹/L. What is the most likely underlying cause?