It has nothing whatsoever to do with fracking. Radon is a by-product of uranium and radium decay. It is present to some level in just about any soil. It is also a gas at standard pressure and temperature, so it is slowly filtering its way up from deeper rocks through the soil and then out into the atmosphere. When you dig a hole, for example a house foundation, you create more exposed soil surface through which the gas can escape. Some aspects of a home also cause more radon to escape into a cellar than would escape through surface soil, like the suction effect of natural convection in the house due to daily heating and cooling. How much radon your local soil is producing can be quite variable due to random variation of subsurface rocks and other effects of local geology.