Supply and demand in the Middle Ages

So many serfs or villeins had died of the Black Death that crops lay ungathered, and fully a third of the arable land was left totally uncultivated. Naturally, the remaining workers appreciated the fact that their services were worth more than hitherto. They became more vocally discontented with the demands of the manorial system, and some refused to work at all on the old harsh terms. The owners of the manors, faced with danger of having vast fields lie untilled, and of the consequent loss of income, began gradually to alter the old manorial terms.

(from A History of England and the British Empire)