When I was doing my initial research a year or so ago, I found quite a few similar stories on various reef forums. I came to unofficially name it SAD (sudden angler death). Many people experienced perceived healthy fish dying out of nowhere. Some guesses were, gluttony, where the food in the gut didn't fully digest and caused the fish to rot from inside out. Also, some anglers need help burping. If an angler swallows an air bubble, at times a person must intervene manually and rub / gently squeeze the fish and burp out the air bubble. Most people probably wouldn't be able / willing, but it does happen.
The final answer is that it had parasites. Sometimes hidden parasites can affect one fish, but as long as the other tank mates are healthy and have good immune systems, they won't be affected.
Good luck in the future with your next one. Still waiting for the right time for myself to acquire another.