I take dents out of 4th gen Camaros and Trans Ams quarters all the time. They get a small crease directly above the center of the rear tire. It has to do with the body twist and torque. If you drive the car gently, you won't get these. Or, you can drive it like you want to and just get the dent removed from time to time. I'm sure an improved suspension could fix this. So yes, the dents do just happen by themselves with certain cars.
2014 Mustang GT Premium: Ruby Red, GT500 exhaust/valance/spoiler and Roush Upper/Lower grills and splitters
1987 Camaro IROC-Z: 350, TTops, Leather, Bose and only 20k original miles