Why does slow blood flow predispose to coagulation (thrombosis)?

This is a little controversial, but the following answer comes from "Textbook of Medical Physiology" by Guyton and Hall (10th edn.):

Blood often clots when it flows very slowly through blood vessels because small quantities of thrombin and other procoagulants are always being formed. These are generally removed from the blood by the macrophage system, mainly by the Kupffer cells of the liver. If the blood is flowing too slowly, the concentrations of the procoagulants in local areas often rise high eough to initiate clotting, but when the blood flows rapidly, the procoagulants are rapidly mixed with large quantities of blood and are removed during passage through the liver.

Hope that helps!