Fire ants are attracted to moisture. If you have wet spots of dirt or soil, they'll set up their colony there until it dries out, at which point they may move into your home.
Here we have an example of a fire ant colony that's moved in underneath the roots of this plant. The home owner keeps this plant and this whole bed more moist then the yard generally speaking, because he wants to keep the plants alive and the flowers blooming. Problem is is that really attracts fire ants from the many different locations around the home. They're all throughout this area. The soil is very loose and allows them to set up camp and bring their eggs to the surface so the sun will warm them. If this area dries out or if the colony splits, the new colony will bud off, possibly move into the house for additional moisture.