Loose wheel nuts can occur for a variety of reasons, including undertorquing, overtorquing, differential thermal contraction, and improper mating surfaces.
Both undertorquing and overtorquing are common when air impact wrenches are used because the actual torque that gets applied depends on so many variables. A common approach is to purposefully overtorque wheel nuts, in part to combat this variability and in part based on the reasoning that "more is better". However, overtorquing actually reduces (not increases) clamping force in many cases, by stretching the studs or threads beyond their ability to respond - especially when this is done repeatedly. Overtorquing can also cause other problems such as cracked, seized, or cross-threaded nuts (which cannot apply the appropriate clamping force), and increases the frequency of stud failure and cracked wheels.
Differential thermal contraction can occur when wheels are mounted at shop temperatures in cold climates. As the wheel components cool to ambient temperatures, clamping force is lost. Wheel nuts which experience any rotation (i.e. back off) during this time will not regain their original torque / clamping force values.
Improper mating surfaces include both damaged and contaminated mating surfaces. Proper clamping force cannot be achieved with non-flat mating surfaces such as damaged or bent hubs and wheels, or worn or elongated bolt holes (raised metal). Contaminants such as excess dirt, sand, rust, metal burrs, and paint on mating surfaces can wear away with use, causing a settling effect. When present on the threads or between a nut and the wheel surface, these contaminants can also change the clamping force / torque relationship, resulting in "false torques" where much of the torque applied is used to overcome friction and is not converted into clamping force.
A loose wheel nut can originate from any of these sources individually, or more probably, from a combination of these sources - which makes the task of eliminating all loose wheel nuts very difficult.