Probably "old age" .. then too bettas develop dropsy usually without any reason. Personally I think it is the inbreeding that shortens their life. If you can get a betta that is of the "wild" type, they are not as colorful but they seem to live longer. They also seem more active. Also, they are real pigs when it comes to eating and have a tendency to become constipated. I feed them once a day . . .and if you feed them live worms, feed only 1 or 2 worms.
Good luck with your next betta.