Before the New Era, thousands of years of human civilization depended on "collective consensus" to uphold every rule: we all believed that 1 + 1 = 2, that walls were solid, that the dead could not come back to life, that day and night alternated, that memories were real, and that logic never changed.
When an unheralded, worldwide mental disturbance struck, the stability of humanity's consensus was permanently shattered. Once the doubts, fixations, delusions, and lies of individuals formed a consensus among a small group, they could rewrite local reality.
Survivors called this disaster Collapse Day. There were no explosions or flames. People all over the world simply began to "disbelieve in established reality" at the same time, and the logical framework of the old world fell apart layer by layer.
