Ventricular fibrillation is most often caused by sudden occlusion of the left anterior decending coronary artery by delamination of an atheromatous plaque or develelopement of a clot in that artery due to nonlaminar flow of blood. In other words,..a suddend occlusion of that artery. Also ischemic heart disease is probably one of the most common causes. Drug toxicities such as phenothiazine toxicity. Other causes might include mitral valve prolapse (more common in females). Profound hypothermia can also cause sudden ventricular fibrillation. Sudden rupture of a papillary muscle can cause this condition.
All result in this same ventricular dysrhythmia and sudden death.