What causes varicose veins? Varicose veins are mostly due to weakened or ruined valves during the veins, leading to blood pooling as well as the veins turning into swollen and twisted. Deep Vein Thrombosis: This happens when a blood clot is made in the deep veins and generates a blockage. https://www.sgvascularctr.com/varicose-vein-treatment-in-singapore-everything-you-need-to-know/