Well, it’s simple really. The structure of their eyes (moths, mantises and plenty of other bugs) is distinctive. Under the microscope their eyes resemble a bunch of long tubes. So when the moth or mantis encounters an artificial light, suddenly when it swoops by the light slips out of its field of vision, and it swings round to get it to shine back into the tubes, and at a constant angle. Hence the chaotic, circular flying. They’re attempting to keep the light coming in at the same angle into the tube structure of their eyes.