Satellites and spacecraft fall to Earth all the time. Vehicles in lower orbits get bombarded by small particles in the planet’s upper atmosphere, and that eventually drags them downward. But usually, these falling objects are small enough or shaped in such a way that they’ll burn up safely while re-entering the atmosphere.
The problem with Tiangong-1 is that it’s rather massive. Launched in 2011, Tiangong-1 — or “Heavenly Place” — served as China’s first ever crewed space station. The module weighs nearly 19,000 pounds and it’s pretty dense too. And it’s estimated that around 10 to 40 percent of a spacecraft will make it down the ground. For small satellites, that’s not much. For Tiangong-1, that’s between 2,000 and 8,000 pounds.
With space vehicles of this size or bigger, operators usually have a plan to safely get rid of them when they’ve reached the end of their mission. If a large vehicle has thrusters, it’s possible to use the spacecraft’s remaining fuel to fire those engines intentionally and dump it over the ocean. Or you can send up another spacecraft with an engine to dock with the decay vehicle and plunge it somewhere safe.
But that’s not what happened with Tiangong-1. The space station wasn’t really meant to last past 2013, but China decided to extend its lifespan for a couple of years. Then in 2016, the Chinese Space Agency announced it had lost contact and control of the space station. And its orbit has been slowly degrading ever since, meaning it will ultimately make an uncontrolled re-entry. Or in other words: “We don’t know where it’s coming down.”