In the absence of people, nature reclaims lost ground.
For our entire history, we humans have been taking advantage of our environment and manipulating it to suit our needs. We've built massive cities all over our planet, beating back the forces of nature at every turn. But what happens when we're not there to maintain our dominance? Nature returns, often with devastating consequences for the abandoned remnants of whatever we left behind. This trope comes into play when man or whatever sentient dominant species was around in the first place, has abandoned a location (be it a house, a city, or even a planet) and the natural ravages of plants, weather, animals, and time itself conspire to ruin whatever that society built and return it as close as possible to what it would be if people had never been there at all.