The Continuum Shift (
When Noel inherited the Blue, her unstable thoughts caused all of the possibilities of the world to become scattershot and mixed. Takamagahara began to Observe and manage the phenomena as they used it to get closer to the Blue; in their attempt to seize it, they released Yūki Terumi from the Boundary and used him as their pawn to get closer to it, reason being that Terumi is not effected by phenomena changing. Terumi's goal was to awaken Noel as the Godslayer - Mu -No. 12- and have her destroy the Master Unit, Amaterasu. As well as Terumi, any Observer given their position by Takamagahara were able to stay outside of the phenomena changing; an example being Rachel Alucard.
Simply, the Continuum Shift caused an infinite amount of parallel phenomena because of Noel's unstable thoughts. Each 'shift' through the continuum was caused by Takamagahara as they observed the different phenomenon. The phenomenon that Takamagahara Observed became the canon one, as soon as something appeared or happened that did not work in their favor, they subsequently switched to a new phenomenon in the hope that they would get closer to the Blue.
The Continuum Shift ended as Hades: Izanami seized Takamagahara for herself and made all events sequential.