What is the presumed cause of most aneurysms of the abdominal aorta?

Several theories. Most common belief is that aneurysms develop due to some inherent weakness in the artery's muscular layer due to an imbalance of collagen metabolism or genetic collagen abnormalities. The only independent risk of aneurysm growth is high blood pressure and smoking.
Atherosclerosis. There is increasing interest in genetic defects syphilis rarely marfan' syndrome and other defects of collagen post dissection.