A 58-year-old man with primary varicose veins of the left lower limb undergoes saphenofemoral junction ligation and stripping of the long saphenous vein. On postoperative day 5, he develops sudden-onset severe calf pain, swelling, and a positive Homan's sign. Duplex ultrasound confirms acute deep vein thrombosis (DVT) of the popliteal vein. Which of the following is the most likely mechanism underlying this thrombotic complication?