Hillbilly Horrors is essentially a subset of the Foreign Locations are Scary trope. And it concentrates on the fact that rural places are particularly scary for city folk.
It consists of many subtexts, many of which are loaded with prejudices about what people from the country and small towns are like. But like many scary tropes, it tells us something about ourselves.
Examples of Hillbilly Horror can sometimes be found in only a scene or two, as a Harbinger of Death when a gas station attendant warns of impending doom (Cabin in the Woods). Or it can define an entire movie series, such as Wrong Turn or The Hills Have Eyes.