What are causes of an aortic aneurysm?

Several things. About 5% of men over the age of 65 have abdominal aortic aneurysms. Risk factors include: increasing age, male sex, family history of aneurysm (may increase your risk 4-fold), hypertension, and history of smoking. Dissecting aneurysms can be caused by localized damage to to the artery from hypertension or trauma (deceleration or sheering-type injury) or catheter injury from an angiogram.
Many. Many causes such as high cholesterol, smoking, uncontrolled BP and certain genetic causes as well as sudden deceleration injuries.
Many. "causes" are associated problems that are thought to be the reasons. Remember, though, that association is not necessarily causation. That said, more important is what to do about presence of aneurysm. Since there is weakness in the "pipe", this needs to be evaluated whether treatment is necessary, urgent or elective, or not at all. Seeing a vascular surgeon for an opinion is needed.