I had too much fun with @dean's construct to stop.
  1. Blue collar workers signing the man
  2. Coal workers singing about injustice
  3. Moonshiners singing about booze
  4. Mountain people singing about how city folk can't understand
  5. Country folk signing about the hollow, the foot hills, the trees, or the canyon
  6. Protestors signing about the strike
    Bluegrass is pretty progressive.
  7. Christians singing about Jesus
    Bluegrass is pretty conservative.
  8. Musicians signing about their instruments.
  9. Appalachians singing about hours of the night.
  10. Farm people singing about live stock.
  11. String pickers not singing anything.
  12. Men singing about constant sorrow.
  13. Truck Drivers singing about weed, whites and wine
    Suggested by @sarahschussheim
  14. Folks singing about Kentucky.