As you can see, there are a large number of parameters that need answered to even get started here.
At the risk of sounding like a total a-hole, you will never find this out for sure without a necropsy being performed.
Here are some possible answers:
Cancer.
Organ failure.
Blood poisoning.
Fight injuries.
Accident injuries.
The reverse of drowning. Airing?
Pregnancy complications.
You can start eliminating these, eg no visible injuries, to reduce the set of answers, then get into more specific questions e.g. if no injuries, is there any signs of disease such as growths, fungus, bloating/swelling. You can then use these to create another list of possible answers and start eliminating these if you want to get to a more specific answer. As I say though, without a necropsy its impossible to be sure. These are also expensive and the fish has probably been dead too long now to do one.
Ultimately, the fish is a casualty of the hobby and what you can do best is:
1) check your water stats.
2) do a large (50% minimum) water change (more if your stats are all over the place).
3) monitor your other fish regularly (2 times a day) for changes. Keep a small journal detailing your observations, makes it easier to make comparisons.