What are the causes of widening of the ascending aorta?

Varies. Hypertension, aortic valve disease, genetic or hereditary causes, certain infectious and inflammatory diseases are among possible causes and risk factors for ascending aorta dilation and aneurysm.
Aorta. The aortic diameter is related to genetics and blood pressure in most people. High blood pressure can cause the aorta to be larger as can some arterial degenerative diseases.