Unfortunately you may never know. I was told by a large aquarium once that even though they did autopsies on every fish that died they still couldn't find a direct cause of death on a very large number of those fish. Even with people it's hard to determine what's wrong when we're sick and we can tell a doctor how we feel. From the information you've given I'd have to say I have no idea what happened. It's just a hazard of the hobby. Sometimes you know just what you did wrong that killed a fish and sometimes it has a stroke or an aneurysm or cancer or who knows what parasite or disease that you never saw. Sorry to hear about your loss
Also, most eels do better on less frequent feedings but larger meals, i.e. feed them until they refuse food then wait a few days to a week (or more in larger eels) and repeat. I'm almost certain your feeding schedule played no part in the eels death so don't worry about that.
If you still have the eel cut it open and see if the shell is there. Odds are it wont' be. I've never heard of an eel eating anything it didn't want to. With their pharyngeal jaws they can be pretty choosy. It'll give you some peace of mind in any case.
