The dragonflights may have been created at the same time, but for the thousands of years they've existed on Azeroth, they've hardly been friendly with one another. It started with the Black Dragonflight and Neltharion's betrayal during the War of the Ancients. In the moment that Neltharion took the name Deathwing, in the moments thereafter during which he destroyed nearly all of the Blue Dragonflight with the Demon Soul -- in those moments, the dragonflights were introduced to a new concept: deception. It was unthinkable that any dragon would deliberately seek to harm another, and yet it happened.
The fallout was immediate. Malygos, driven mad by the betrayal of one of his closest friends and the loss of his flight, fled to Northrend. In his madness, he split the Nexus from the rest of the land, separating Coldarra from the rest of the Borean Tundra. And then he stayed there, alone in his despair and insanity, refusing all visitors in his grief. The Blue Dragonflight crumpled, held up only by those who stood and tried their best to hold together the shattered remnants of the flight.