It took me a few minutes to think of the word ... ''teleology''.
From wikipedia: A thing, process or action is teleological when it is for the sake of an end, i.e., a telos or final cause.
I remember that teleology is a really big no-no in the scientific worldview, but I can't recall the context (perhaps evolution?) and the arguments against it at the moment. Without remembering the first context, and not having at hand the generalized shape of the fallacy, I'm about two steps away from owning what the error is.
The context is not just evolution but naturalism in general.
The teleological fallacy, found in religious and "traditional" world-views, says that most or all things exist for a purpose. By examining how they are useful to us, or to other living creatures, we can often discover this purpose. The purpose is an inherent and unchanging quality of things, which also means there are objectively 'right' and 'wrong' ways to use certain things, 'natural' and 'unnatural' behaviors.
Examples: hares exist for foxes to eat. The Sun exists for us to be able to see. Air exists for us to be able to breath it.
This is, in fact, a triple fallacy. First, it often assumes (or, worse, tries to prove) a creator God that created everything for a purpose, which is false. Second, it reverses the direction of adaptation: in reality, given that there is a sun, we evolved eyes that can see its light; subterranean creatures lose their eyesight. Third, it claims to find objective, inherent, unchanging purposes in things (and thus, define 'natural' or 'correct' behaviors), but in fact these claimed purposes are entirely subjective either due to the choice of the method for finding them, or (in many cases) due to people simply announcing what they think a thing's purpose is without any method at all.
Ha. I remember now.
The fallacy is anthropomorphism, because only beings with intention and planning can do something 'so that..'. For example, I can save money for college so that I can get a degree. Even a cat can wait by the bird cage 'so that' it catches the bird if the bird ever escapes. However, water can't drip through a rock 'so that' it makes a stalactite.
The author is making this mistake of teleology if he explains events occurring so that they break down civilization; if he describes a model whereby the system was ready for the disruption to occur and then events happened so that it would.
For example, saying that an earthquake occurs 'to release energy' is such a conceptual mistake. The earthquake event is related to the build up of energy, and the release of energy is a by-product of the event, but the earthquake did not occur in order to release energy. That is the teleological mistake. (The author might have made this mistake, or not, I'm just relieved I remembered what was tickling my brain after reading this post.)
You can define a category any way you please; the question is, what useful attributes and predictions are common to all members of the category. I either don't understand or don't agree with your distinction between categories of causes and of effects.
For instance, if you can claim that all things causing effect Y also have a common attribute X, that's a legitimate category that makes a useful prediction - even if there's no single mechanism underlying the relationship.
The real problem with the argument you quote seems to me to be that the statement "there are two forces that act on humanity" is not useful. It's either tautological ("various things have happened in history; some of them have brought us together, and some others have driven us apart"), or meaningless and/or wrong ("everything that ever happened to humanity either brought us together or drove us apart") or just useless ("both the fall of Rome and the Black Death drove us apart, for a suitable definition of "drive us apart", but that's the only thing they had in common").
Describing the Black Death as a force that breaks down civilizations seems a bit far fetched, since the Renaissance started not too long afterwards in Italy (which was among the hardest hit). A good case can be made that the Black Death was one of the causes of the Renaissance.
(Of course I don't know what the actual argument of the author was, he may have addressed this fully.)
I sorta doubt this is the guy's point, but I think you can think of such a force. You can think of this hypothetical force as an entropic kind of force, there are many plausible ways for a civilization to break down than for it to continue or grow; being a civilization just makes you vulnerable to many kinds of problems. In order to maintain a civilization, you need a different force for larger civilization to counteract this hypothetical force.
In the absence of information about the book's author or topic, I think I'd be most inclined to see that as a semi-original formulation of good and evil rather than anything trying to be particularly scientific. Now, I suppose you could interpret that as a model of reality (when I stub my toe, there are malign forces at work), but it's just as likely to be used for reasons other than accurate modeling. Emotional resonance comes to mind.
Recently I heard an author interviewed on NPR about his book. I can no longer remember the book's name or topic; but I remember that a couple of times during the interview, the author made puzzling categorizations. One of them was approximately: "Throughout history, there are two forces that act on humanity: One bringing us together into large civilizations, and one that breaks down these civilizations, like in the fall of Rome or the Black Death."
The author probably thought he was being scientific in perceiving patterns. But someone who takes the fall of Rome, and the Black Death, and says they are both manifestations of a single force, is doing mystical science. Science says that things with the same underlying causes form a category. Saying that things having the same effects form a category is mysticism.