A 52-year-old man with a 20-year history of alcohol use disorder presents to the emergency department with haematemesis and melaena. On examination, he is hypotensive (BP 85/50), tachycardic (HR 118/min), and has clinical signs of chronic liver disease including spider angiomata, palmar erythema, and ascites. Laboratory investigations reveal INR 2.8, platelet count 65,000/μL, and total bilirubin 3.2 mg/dL. Upper endoscopy confirms bleeding oesophageal varices. What is the most likely underlying pathological mechanism responsible for the variceal bleeding in this patient?