The short answer is that it's an analogy to attractors in dynamical systems, where a wide "basin" of initial conditions tend to evolve toward a narrow region of attraction.
It's funny that you say that is "an analogy", because physics is also "an analogy" then, just more precise.
I ask where it originated because I've seen that there's a concept of "cultural attractor" in papers of anthropology and sociology that maps well with this notion, so I thought that there would probably be a direct causal connection between the two instances of the concept (from academic study to LW), but I haven't seen anyone mentioning that nor the expression "cultural attractor" being used.
I initially thought that it was just the local jargon, so I got surprised.
I've seen the expression being used in some posts to refer to social structures, like in cultish countercultishness from Yudkowsky.
I have a vague intuition of what the idea is about but I would like to see a formal explanation of it, and where it originated.