This was, quite simply, the end of the world.
Or, at least, the Pilot.
Its cause is still under dispute; the leading theory seems to be that the Count of Saint Germain's defeat of the Watcher disrupted the order of the universe, and that either the universe disintegrated and was replaced, or the Improbability Drive exerted a special effort to rewrite existence to make it so that it had never happened. 1)
Whatever the case may be, the approach of the Vortex was sensed first by a few especially time-sensitive and Improbable contestants due to the pull of its rising flux on both spacetime and Improbability. By the end, it was so strong its echoes manifested in normal spacetime, clearly visible to even the most mundane contestants as a mass of churning dark clouds that blanketed the Island, full of weird flickers and distortions, the eye of the whirling storm situated directly over Improbable Central.