Here in Beijing people also wonder how it happened, some common rumors say that the government planted so many trees to have formed a very tight green belt around Beijing to protect her from sandstorms coming from Mongolia Gobi desert and then the stand storms were gone but came in with the smog instead, because the trees stopped the free air flow and the wind can not come into Beijing to blow away dirty air. Now the government cut down many trees and make an open passage way through the man made forest to allow wind coming in and the wind did the trick. But to me that's total bullshit, sounds more like urban legend to me.