Florida
Best Decade: 1970s (+4.20 Seats)
Reason: IDK
Worst Decade: 1910s (+0.43 Seats)
Reason: Maybe the state was seen as a backwater back then
Georgia
Best Decade: 1990s (+1.32 Seats)
Reason: The state at the time was seen as some kind of fundie zion
Worst Decade: 1920s (-1.58 seats)
Reason: IDK
Mississippi
Best Decade: 1970s (+0.10 Seats)
Reason: IDK
Worst Decade: 1910s (-1.13 Seats)
Reason: IDK
South Carolina
Best Decade: 1970s (+0.46 Seats)
Reason: IDK
Worst Decade: 1920s (-0.75 Seats)
Reason: IDK
Virginia
Best Decade: 1940s (+0.77 Seats)
Reason: boom in wartime industries?
Worst Decade: 1920s (-0.90 Seats)
Reason: IDK
Not certain about Virgina, but in the other four states, the reason why those were their worst decades was certainly because of the boll weevil.
The boll weevil crossed the Rio Grande into Texas in the 1890s and spread. It devastated the cotton fields, killing off sharecropping as it killed the cotton, and was one of the principal causes of the Great Migration of Negroes from the rural south to the urban north.