Currently, the diagnosis and treatment of an aneurysm are informed by correlative guidelines on its size and growth rate, along with a holistic consideration of associated symptoms and pathologies. However, aneurysms can exhibit significant growth or rupture before these criteria are met; conversely, large aneurysms may exceed intervention criteria but remain stable over time. Without methods to evaluate the key factors driving aneurysm development, it is difficult to assign preventative treatment to patients most in need. In this work, we propose a fluid-structure instability that may cause blood vessels to dilate and form aneurysms, or induce existing aneurysms to grow significantly. The ab-initio framework yields a stability parameter that describes the onset of this "fluttering" of the aortic wall, which may drive or signal the presence of abnormal aortic growth. A retrospective study was conducted with 4D flow MRI data from 117 patients indicated for cardiac imaging and 100 normal subjects recruited prospectively as healthy volunteers. Both cohorts comprised subjects with magnetic resonance imaging (MRI) scans of the full thoracic aorta. The stability parameter was calculated for every subject from the earliest MRI data. For a 72 patient subcohort with at least one follow-up imaging exam after the initial MRI scan, the growth of each patient's aneurysm was forecast by the stability parameter and compared to the maximum growth rate observed in follow-up as well as any subsequent surgical intervention. A receiving operator characteristic (ROC) curve was used to gauge the stability parameter as a potential diagnostic physiomarker. The area under the curve of the ROC analysis was 0.97. The proposed flutter type instability appears to offer accurate prediction of aneurysm development from patient specific data that can inform precise, targeted management of disease progression.