Your definitions are wrong. Agreement and centralization are not related like that. If anything, it would be the other way around: there's agreement in an artificial decentralized order, since all participants agree to the rules and join, and there's not necessarily an agreement in centralized order, since the order may just be defined centrally and imposed to all participants. (There may be centralized organizations born out of agreements too, though).
Decentralized orders may be born of common agreements among its participants. They may also rise spontaneously (naturally), when some conditions are met, like natural evolution or the free market.
second that. Agreement comes from the need to cooperate. Another example is language. Why do you think people settle on only a few languages globally? Nobody "tells" them. It's doing you no good to learn languages which nobody else speaks....