Li0nFish, I'm sorry to hear about the loss of your guppy. Dropsy is a common problem in fancy guppies, and as Avery mentioned, it is usually caused by poor genetics. Unfortunately, there is not much you can do to prevent it.
Overfeeding can contribute to dropsy, so it's a good idea to monitor how much you're feeding your fish. You could try feeding them smaller portions more frequently throughout the day instead of one big meal. Also, make sure you're not overstocking your tank as this can lead to poor water quality, which can also contribute to dropsy.
Keep an eye on your remaining fish and watch for any signs of illness. If you notice any bloating or other symptoms, it's best to quarantine the affected fish and treat them with medication.
I hope this helps and that your other fish remain healthy.