It is hard to lose one especially when you have no idea why. I had a EE female in my sorority that up and died one day for no reason. Never did figure out the problem because no one ever got sick after that. I had one girl get columnaris but got her out before anyone else was affected thank god.
Your chunky one might have had parasites that never showed themselves. That is something that happens quite frequently as well which is why a lot of people have started to treat the fish for parasites when they first get them with or without symptoms.