It seems to have started, if you look at the graph in this NYT article, before any economic problems hit them: employment was high, wages were rising. But they have increasingly been dying from suicide, drug overdose and alcohol-related problems like cirrhosis. Experts are puzzled, but they do note that:
One thing I wondered was why there was no mention of nutrition or obesity in the article. They should have talked to Michael Pollan. Smoking is still rampant among that group as well, although I’m not sure if it has actually gone up since the mid-'90s. Any other hypotheses?