Most common cause of an aortic aneurysm is -
An aneurysm 

An aneurysm is a localized abnormal dilatation of a blood vessel or the wall of the heart.
When an aneurysm is bounded by the true arterial wall, it is called a true aneurysm.
When wall is ruptured and there is a collection of blood outside the vessel wall (extravascular hematoma) that is freely communicating with vessel wall false aneurysm. Most common cause of a false aneurysm is posted myocardial infarction rupture.
An aneurysm due to an infection that causes weakening of vessel wall → mycotic aneurysm.
The two most important causes of an aortic aneurysm (not only abdominal part but in any part of the aorta) are atherosclerosis and cystic medial degeneration of arterial media.
Syphilis
Marfan's syndrome
Atherosclerosis
Congenital