Unfortunately it could have been anything that killed it. It might have had a heart attack or stroke, or there was a small ammonia spike that killed it and has since cleared up. Sometimes fish just up and die and the only real way to find out what killed them is to do an autopsy. But that costs a bit of money so most people just accept the loss. However, it would be a good idea to monitor the rest of the fish over the next few months and keep records of any that die, including what they looked like and water quality. Also keep photos of the dead fish. Then if you start losing fish on a regular basis (every months or so, and they have similar symptoms, you can try to work out what is killing them from the history.