It is extremely rare for atrial fibrillation to develop in younger patients, unless they have an underlying heart condition. Experts say there are many cases in which the cause of the atrial fibrillation was never found. Here is a list of the most common causes: 1  Hypertension-high blood pressure. 2  Coronary artery disease-also known as coronary heart disease. 3  Plaque builds up inside the coronary arteries. 4  These arteries supply the heart muscle with blood that is rich in oxygen.