One of the problems in the days of James was the mistreatment of the poor by the rich. The rich would treat any way they wanted those who provided a service for them. James says the rich must treat people fairly (James 5:4-6).
James tells the rich they should not be withholding the wages of those who “mowed [their] fields” or harvested their crops. In both situations, the service providers were crying out to the Lord against the rich.