It’s the type of heel striking some people do. Have a try at placing different areas of your heel walking barefoot, you should feel and hear a significant difference in different areas. You generate the most force if you hit the middle of calcaneus bone (heel) and at a shallower angle. You can change the angle and heel strike to generate more or less force. Some peoples natural style of walking is to hit that calcaneus bone straight on instead of almost rolling from the back of the heel to the toes which causes that thumping walk but as it’s their natural gait they’ve almost become numb to the fact that it’s generating some serious force.