I wonder if the potential oil boom in North Dakota might end some of the longer streaks of decline up there.
I suspect it will.
One interesting discrepancy is with the Rust Belt- the major urban industrial centers started declining in the 1970's and smaller ones in the 1980's, but the big mining and lumber counties were seeing their populations drop 20 to 50 years earlier.
Other obvious historical effects that show up are the farming crash and Dust Bowl in the 1920's-1930's, a number of mining towns the went bust in the west and never came back, the Great Migration and Second Great Migration in the south (a bit harder to follow since the Great Depression fell in between and also hit a lot of Southern communities hard), suburbanization of northeastern cities in the fifties, the decline of tobacco country in Virginia, but only there for some reason as central Kentucky and North Carolina are going strong; and Hurricane Katrina on coastal Louisiana.