The trigger for that is epigenetic, that is, it's caused by genes but they aren't always active. There's a "blocker" molecule on the DNA that prevents those sections from being used for making proteins. When certain things change in the environment, you can gain or lose those blocker molecules.

One of them controls fat storage, and activates during starvation to encourage your body to hold onto any nutrients it can get. Because it is genetic though, it's inheritable. So if any of your parents or grandparents went through a famine or a time of starvation, the epigenetic marker could have been removed and you inherit that missing marker, causing your body to store fat more than other people who have the marker still.