Possibly an internal infection that damaged the fish's intestines and it expelled everything (stomach, intestine) into the tank. It might have been a female that expelled eggs and died due to complications.
It could also be coincidence and there might be other factors like poor water quality that killed the fish.
How long has the tank been set up for?
How often do you do water changes and how much water do you change?
Do you dechlorinate the new water before adding it to the tank?
Do you gravel clean the substrate when you do a water change?
How often and how do you clean the filter?
It's a good idea to do a 75% water change and gravel clean the substrate every day for a week if you ever lose a fish. This dilutes anything in the water that might have harmed the fish or been released by the dead fish, and will reduce the risk of the other fish developing the same problem.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank. The best way to do this is to fill up a clean bucket with tap water. Make sure the bucket is only used for fish and nothing else. Add the required amount of dechlorinator to the bucket of water and then aerate the mixture for at least 2 minutes (but preferably 30 minutes or more). Then use that water to fill up the aquarium.
If the tank has been running for more than 2 months then clean the filter if it hasn't been done in the last 2 weeks. Wash filter materials in a bucket of tank water.
The following link has some information about what to do if your fish get sick. It's long and boring but worth knowing. I recommend printing it out and reading it in bed to help fall asleep.
http://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/#post-3804819