What is the typical underling cause for aortic aneurysm?

Arteriosclerosis. Most arteriosclerotic aneurysms result in cystic medial necrosis of the aorta resulting in dilatation of the artery secodary to the internal pressure and weakness of the arterial wall. Any part of the aorta and its branches may be affected. Infections with bacteria, fungus, syphillis may also produce aneurysms in the aorta.
Lot of things. Atherosclerosis (hardening of arteries), aging, infection (syphilis for one), traujma, some congenital problems such as marfan's or ehlers-danlos where there is inherited weakness in the artery wall, and inflamation. Hypertension can cause a "dissecting aneruysm" (a tear in the aorta).