Shockwaves in an Asteroid Apocalypse

It's not impact, but the shockwave. All I can say is, it makes sense. Heck, a large enough asteroid can just graze our atmosphere, but if the shockwave and resulting "splinters" are enough, then we're in trouble.

Source:
http://gizmodo.com/we-were-wrong-about-how-the-asteroid-apocalypse-will-go-1792727736