What are some causes for sudden death in adults?

So many. Commonly a blocked artery can cause sudden death, but also an old heart attack that caused scarring of the heart muscle or weak heart can predispose to sudden death. Also sudden death can occur in patients with genetic abnormality that cause the heart to be enlarged. Sudden death can occur with anomalies of coronary or with genetic abnormality that cannot be detected by echo or EKG.
Familiar. Coronary artery atherosclerosis, either severe or with an unstable plaque. Heart rhythm problems from drug use, inborn ion channel errors, hypertrophic cardiomyopathy (i don't accept "big heart" as fatal), myocarditis. Pulmonary thromboembolus. Missed addison's. Pulmonary hypertension can kill suddenly as soon as the right ventricle. Rapid but not instant death common in brain bleeds, asthma.