Inspired by PT and by conversations on a couple of other threads:
Reality, or at least Harry's particular universe, has a "threshold" similar to a home. Mortals can summon Outsiders (but supernatural beings can't) for the same reason that mortals get thresholds and supernaturals don't: the "magic mechanics" is the same as the threshold/invitation principle.
(In this analogy, Winter's job is to keep the Outsiders from just "burning the house down".)
Certain things can weaken the "threshold" of reality, in the same way that Sells' rituals wrecked the threshold of his lake house.
And it waxes and wanes on a cycle, similar to how the Nevernever-mortal world boundary is weakest on Halloween (as seen in GP and DB). The weakest point is coming, and this is what will cause the BAT.
The "starborn cycle" of every 666 years is a "defense mechanism" for reality that's part of the same cycle; the starborn conjunction occurs maybe 40 years before the weakest point, in order for that cycle's starborn to be trained up and ready to fight when the Outsiders are strongest.
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I think the actions Harry attributes to the "Black Council" come from two different groups with different goals: Nemesis and the Nemfected on one hand (who want the Outsiders to win), and the Circle and the Fomor, who want to profit off the chaos (so they want the Outsiders to almost win, but ultimately lose) on the other hand.
Due to the "mildew" descriptions of Cowl's Nevernever gates, I think the Circle is working with the Fomor. Ethniu is attacking Chicago in sync with an Outsider assault on the Gates, so they are clearly working together at some level, but I doubt Ethniu wants reality destroyed. Both sides (the Circle/Fomor/Ethniu and the Outsiders/Nemesis) probably believe they are using the other for their own ends.
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Further WAG: The specific goal of the Circle/Fomor alliance is to let the Outsiders weaken reality enough that they can rewrite it to their own ends via magic, gaining essentially absolute power, and then kick out the Outsiders. Lord Raith and Maggie Sr (before her redemption) were members, and Harry was originally intended to be the Starborn that implemented the "kick out the Outsiders" part of the plan.