The gate is hidden deep within Commorragh and its existence known only to a few. So I wouldn't say it magically appeared, just information came to light. Thing is though, the Covens mess with psykers all the time. Take 3rd ed' soulseeker ammunition for example. Or as we've been discussing, Crucibles. Anyone read the Dark Eldar series? There are 3 diddy Talos's made out of psykers that telepathically communicate to hunt their target.
Pain is the only universal constant. Pain is all. It is the key to creation and destruction both. Thus does he who masters pain become a god.