The reformation was an event in Drayan history.
Once brilliant scientists and engineers, they had been continually developing better, smarter, more efficient machines, until the technology became more important than the people. The reformation ended that path, and instead they "returned to themselves". Since that time, they remained isolated to avoid the influence of those who might lead them back down the wrong path. The event was similar to actions taken by certain Human cultures.
Alcia, the great-granddaughter of one of the people who helped usher in the reformation and the Drayan leader during the 24th century, believed that without the reformation Drayan society would have self-destructed. (VOY: "Innocence")