Can you tell me why there are often life-threatening aortic defects associated with marfan's syndrome?

Yes: Aortic dissection is a known complication.

Firbrillin 1: In short, problem in Marfan's is a misfolding of a protein involved in the structure of connective tissue in various parts of the human body, notably in tissues such as mitral and aortic valves and aorta, which can lead to mitral valve prolapse, aortic dilation and aneurysms, aortic regurgitation, aortic dissection, etc along with extra cardiac manifestations.