The author being under the impression that everything has to originate from, pass through, or other wise get connected to Earth in some fashion. Be it transplanted Earth people, duplicate Earths etc. That is if you stand on Earth long enough every race in the Galaxy will pass before you. Star Trek is particularly bad for this. One wonders how ancient man developed "normally" while tripping over aliens all the time. Also known as "It's a Small Universe" [Garry Stahl]
A second meaning is in using one McGuffin as the cause to too many things in your universe. Life is seldom that tidy or consistent. My "favorite" in this category are the Rishans, that get blamed for everything from the Breakup of the Beatles to the Fall of Man (I suppose we can add Obamacare to that), metaphorically speaking, or literally, I don't know. The point being; vary the source of great events.