There is a substantial place in social causation for mechanisms that link the intentions of powerful actors to the specific features of the outcome. "The outcome came about because the powerful actor wanted it to." Why are there no petroleum refineries in mid-town Manhattan? Because zoning and planning boards have deliberately excluded such activities.
But what about causal mechanisms that are not the result of strategic choices by social actors? Are there impersonal social causes?
There are rare but real instances of social changes that occur without any intermediary of social action -- for example, the eruption of Mount Vesuvius and the extinction of Pompeii. But these events fall outside the scope of the social sciences. And there are important social explanations that begin in impersonal features of the natural environment -- for example, the configuration of rivers in China's early history. But what makes these into social explanations is the analysis of the social behavior through which agents adapt these conditions to their needs. (See Mark Elvin's truly excellent environmental history of China for more on this; The Retreat of the Elephants: An Environmental History of China.) But social explanations always involve actors -- and that means that intentional social action always comes into the picture in some way. So we might begin by saying that there are no impersonal social explanations, if by that we mean "explanations of social outcomes that do not involve the actions of persons."
It is important to observe that there are actually two distinctions that are relevant here. There is the "personal-impersonal" distinction, and there is the "intended-unintended" distinction. In an obvious sense all social causation is "personal", in the sense that social causal mechanisms are always embodied in the constrained actions of socially constituted actors or persons. So the actions of deliberate actors are part of all social causation. But the intentions of the actors are often unrelated to the social outcome we are trying to explain. So in these cases the outcome is not caused by actors' intention that it should come about. In the refinery example -- it may be that there is no regulation prohibiting this kind of activity, but the cost of real estate makes the proposition unattractive from a cost-benefit perspective. On this scenario we would have the result occurring as an unintended consequence of the choices of a large numbers of independent actors.
These are the most interesting social explanations: explanations of social patterns or outcomes that are not the result of design or intention, but that nonetheless emerge through the purposive actions of large numbers of agents. These are "unintended consequences" explanations or "aggregative" explanations. We can quickly identify dozens of such examples: the silting of river deltas as a result of flood-management strategies upstream; the expansion of black-market sales of cigarettes as a result of new taxes on tobacco; the expansion of traffic flows as a result of the opening of the third harbor tunnel in Boston; etc. These explanations are "aggregative" in the sense that they work by "aggregating" the lower-level choices and preferences of individual actors into a higher-level social pattern. (Thomas Schelling offers numerous intriguing examples along these lines in his book, Micromotives and Macrobehavior.)
So now we can answer our original question. There are no social causes that work entirely independently from social actors, and actors are purposive. So all social causation stems from "intentional" human behavior; persons are always involved in social outcomes. However, there are many social outcomes that are unintended and unrecognized by all the participants. The participants' intentions are local and parochial; whereas the social outcome is large and unforeseen. These instances are the most interesting problems for social inquiry. We might refer to these as "agency-based explanations of unintended and unforeseen outcomes."
This suggests a different way of classifying social causes: outcomes that are the intended result of specific powerful actors (conspiracy, leadership, dictatorship); outcomes that are the result of strategic interaction among a small group of purposive agents (bargaining, collusion, cooperation); outcomes that result from concerted collective action by large groups with some sense of collective goals (boycotts, strikes); and outcomes that are the aggregate result of uncoordinated but constrained choices by large numbers of independent agents (markets, habitation patterns).
This classification also makes it more apparent why the concept of power is central in social explanation. The first three categories imply a distribution of powers across specific agents and groups, in order to account for the postulated connection between the agent's purposes and the eventual outcome. And the fourth category implies the exercise of power by some other agency, to account for the observed constraints on choice that constitute the heart of this type of explanation.