I can only speculate since I know next to nothing about that but AFAIK those thromboses are not in leg veins but in different parts of the body - in the brain (Cerebral venous sinus thrombosis), lungs (embolism) etc.

Also the mechanism which causes this seems to be better understood now - https://www.nejm.org/doi/full/10.1056/NEJMoa2104840

"Vaccination with ChAdOx1 nCov-19 can result in the rare development of immune thrombotic thrombocytopenia mediated by platelet-activating antibodies against PF4, which clinically mimics autoimmune heparin-induced thrombocytopenia."