Why do things (like space debris) fall out of orbit?
They don’t just fall out of orbit. They’re already falling, but with enough forward velocity so they keep advancing around the curve of the planet as to miss it all the time. If something slows them down then they will miss the planet by less, effectively lowering their orbit altitude. If their altitude gets to be below about 150 miles, then there is a teensy bit of drag from the atmosphere and they start losing forward velocity. In the case of the two satellites that collided, they hit each other at an angle so a good part of their velocity was converted into heat, and the remaining pieces lost altitude. Apparently some pieces lost enough altitude to re-enter the atmosphere and land in Texas.
