Kevin Drum does some analysis on the chart above.
My guess is that part of this is a result of the end of the baby boom and the graying of the American population. But only a small part. Almost certainly the bulk of this downturn is due to the fact that the Republican economy of the past seven years has been aimed like a laser at improving the fortunes of the affluent, with the result that for the first time in recent memory an economic expansion — a long economic expansion — hasn't improved the fortunes of the middle class even slightly. After seven years of this, the working and middle classes are finally starting to realize that this isn't likely to change.
The important question seems to be if the working and middle classes realize it, what is it going to take for them to start to do something about it? Do they believe that a Democratic Administration would help? Or have the anti-government elected officials finally collapsed all faith in the possibility of good government?