From "Stargate SG-1: Ripple Effect (#9.13)"
It's possible the first spatial tear may have caused a ripple effect across several realities, transforming an area of their subspace into into a kind of funnel that's redirecting their gate travel into this universe. Any wormhole passing close enough to the black hole is being drawn into the singularity and rerouted here.
So you're telling me that we're the hub?
Lt. Col. Samantha Carter
A convergence point, yes sir. It might account for why gate travel in this universe isn't affected.
Inform all off world teams to proceed to the Alpha Site. Until further notice gate travel to and from Stargate Command will be suspended indefinitely.