The original definition of "conspiracy" circa 1300s, was simply "acting as one" derived from the Latin root "breath together" or to be "acting in the same spirit" depending on the sense of "spire" (which was also the origin of "spire" in the sense of a cathedral's architectural "spire").
Therefore the original definition does not denote conscious intent to act in coordination with others of the same "spirit", as does the modern definition. Somewhere along the line, the connotation of deliberately coordinated action became denotative.
I am going to argue below that this more restrictive denotation of "conspiracy" was a result of a "conspiracy" in the original sense of the word -- a "conspiracy" which did not require any deliberate, consciously intended coordination of action but was, nevertheless, the work of a group (or groups) for whom that restriction of definition was an evolutionary advantage to their selfish genes.
Group selection produces unconscious coordinated action between members of the group -- and humans have been under group selection since our common ancestor to chimpanzees (see E. O. Wilson's "The Social Conquest of Earth"). This has the same quality of coordinated action that occurs in the eusocial organisms -- organisms that also engage in group, as opposed to individual, combat aka "war". Indeed, the world's foremost authority on eusocial organisms, E. O. Wilson, argues persuasively that human society -- particularly "civilization" -- is evolving in that direction, which ends in the reification of the group, itself, as meta-organism -- a group of organisms "acting as one" on behalf of selfish genes expressing in the group's behavior patterns.
Now here's the key:
Because of the great diaspora of the human genotype out of Africa into a wide variety of environments, there has arisen biodiversity in the human genome adapting to a wide variety of population densities. In the areas with higher population density, there has been stronger group selection than in areas with lower population density. Over the tens of millenia, and in particular over the last ten millenia with the rise of agriculture, this has led to a substantial increase in the gradient of genetically adapted group cohesion between groups. Because these groups were not mixing, due to limitations in transport and barriers of language, natural adaptation to climate, as well as "xenophobia", this didn't immediately result in the destruction of the more individualistic populations.
However, with the rise of empires and resulting mixing of widely dispersed populations, it became a decisive factor in human evolution.
The original definition of "conspire" allowed more individualistic populations to talk about perceived patterns of behavior that were of vital interest to them, without taking on the burden of proof that there was some sort of conscious, secret Cabal behind the pattern. This burden of proof was advantageous to the unconsciously coordinated group organisms since it was, of course, impossible for the individualistic populations to bear in their attempts to come to grips with what was happening to them.
The most recent and stark example of this is in the mass rapes occurring in Germany where there is a "conspiracy theory" that the refugees acted in a conspiratorial manner to have some of them creating diversions while others engaged in rape of German women. There is no need to posit conscious intent on the part of the "rapefugees" and there is reason to believe they may be from populations more adept at group conflict -- unconscious warfare -- than others.