It is hard to look at this data and see anything but a positive story, but apparently the New York Times and the rest of the media only see tragedy. If there is no problem, there is no justification for increased government power, therefore there must be a problem.
(I am presuming this is in real dollars rather than nominal, but God forbid that the NYT ever makes such things clear). They do manage to show a slight negative recent trend in the growth of the percentage of low income Americans, but only by cherry-picking the dates of comparison to the peaks and troughs of the last two business cycles. Overall I would read the story as middle and lower class are moving into upper income brackets, but the Times headlines it as "Middle Class Shrinks Further as More Fall Out Instead of Climbing Up," illustrated with a classic empathy-inducing sad-mom photo.
By the way, since more rich people fall than middle class, it would seem to make sense to discuss instead the falling fortunes of rich people, but of course the NYT has no desire to write that article.