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The idiom, '
waiting for the next shoe to drop' means waiting for the inevitable consequence. The idiom comes from the old days of thin hotel or apartment room walls, when one resident could hear the next door neighbor taking off his shoes before bed. The sound of one shoe tossed aside meant that the sound of the other shoe would soon follow.
In your link, the inevitable consequence or next event to follow upon the subprime crisis will be that even people with good credit will have difficulty getting mortgages.