What are possible causes of most aneurysms of the abdominal aorta?

Atherosclerosis. Some theories about collagen, ischemic wall etc, but most are in atherosclerotic vessels in abdomen or elsewhere. Aneurysms can happen in any blood vessel, but AAA is the most common. Some rare infections , congenital, traumatic.
4 things. Tobacco, diabetes, hypertension & hyperlipidemia... Are the primary causative factors... Genetics is a distant secondary cause...