The Collapse of the Warp Gates

Unlike Earth which suddenly found its long period of isolation brought to an abrupt end by The Cataclysm, the Warhammer world was much closer to the epicentre of the event.
The warp gates which had been constructed by The Old One's millennia before were struck with such irresistible force that they suddenly and completely collapsed. From their position located in orbit over both of the poles of the planet, the gateways erupted into the atmosphere unleashing a wave of raw warp energy from the void into the material environment of the known world and opening permanent portals between the void and the planet.
Although the worst concentrations were nearest the poles, the polar winds spread the warp energy freely across the entire surface of the planet, so that for a while the whole planet was shrouded in it.
However, exposed to the laws of the material universe for the first time the energy from the warp was unable to maintain its ethereal and immaterial form for long, and unable to detect any of the familiar psychic emanations from the Eldar on the Warhammer World to consume it latched onto any psychic references it could detect to provide it with both a substance to manifest into and a source of power in the material plane.
All manner of strange creatures and objects were spontaneously created on the Warhammer planet based upon whatever random thoughts and emotions the warp energy encountered.
The lands closest to the poles were turned in absolute wastelands of chaos where even one's worst mortal nightmares had become reality. But such was the sheer volume of warp energy which initially burst over the planet that not all of it could find a strong source of psychic stimulation and sustenance.
The warp energy that failed to find any psychic reference to shape its existence simply became dormant and solidified into either dust, or stone or a crystalline form falling across the surface of the planet and becoming absorbed into the natural environment.
Whilst at the two poles the initial surge of warp energy following the collapse of the gates has reduced to a varying trickle of fresh energy that continues to seep through the collapsed gates to be spread around the world by the wind.

The world is dying. It is better for you to know it now. You say you live and breathe and love and hate and fear...yes...these are mortal things...and you are indeed alive...but alive on a world suffering from the poison of its death-wound. It rots in the north, spreading like a gangrenous wound. And we...we are but maggots on a corpse. From The Tome of Corruption

A great calamity struck our world. You see, the Old Ones lived in the shadow of the Gate to Heaven, the entrance to the divine realms. Perhaps the Gods grew weary of this first peoples’ pride, or, mayhap, it was fault of their great magic, or even the Ruinous Powers themselves. Regardless, the Gate fell, and so it was that the world was fatally wounded. Chaos swept through the lands, changing and destroying everything it touched. And, so, Chaos has leaked into our world ever since, ebbing and flowing like the tides of the seas. Teachings from the Tome of Corruption