Anything that drags on a spacecraft will slow it down. In orbit, if you slow down, you go into a lower orbit, which causes more drag, which puts you in a lower orbit. Pretty soon you crash back into the planet.
This is what happens when satellites no longer have the fuel to correct their position which decays over time. Their orbit slows so much that the pull from earth is greater than their ability to resist, and they re-enter the atmosphere. This also happened to Skylab and the Mir space station, and close watch is kept especially if the object is large enough - as they were - to not completely burn up during re-entry.