Why did people leave where they lived?

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Across
  1. 2. earliest
  2. 3. conceal
  3. 4. empty
  4. 10. stand for
  5. 11. raise
  6. 14. procedure
  7. 16. to become something
  8. 18. arms
  9. 19. quick
  10. 20. gadget
Down
  1. 1. outdated
  2. 5. collect
  3. 6. cause
  4. 7. joined
  5. 8. field
  6. 9. slowly
  7. 12. get better
  8. 13. thus
  9. 15. shudder
  10. 17. stream