Atherosclerosis
Cholesterol deposits on the inner wall of the vessels, particularly at bifurcation site, then ulcerations occur on these cholesterol deposits
At later stages, platelet aggregants form on top of the ulcerated site of the cholesterol deposits, enlarging in size and causing stenosis of the vessel lumen
When the platelet aggregant grow in size, it becomes fragile and easily can brake and dislodge distally in the vessel causing obstruction in smaller branches. Also if the stenosis increases progressively, it eventually might cause complete obstruction of the vessel.
See this dynamic image illustration
