When the crusades returned, they fled and went to new towns and cities. This increased trade and made it harder for Kings/Nobles to manage. This gave people more freedom and the king/nobles less power
Growth of Towns and Cities
There was a desperate need for workers after the plague, therefore a demand for more money and rights. There were also many peasant rebellions after nobles tried to return to the system. This weakened the feudal system and gave peasants more power
Kings no longer relied on nobels and knights for army after raising one of there own. There was also a change in military technology which reduced the need for castles. The feeling of this nationalism decreased the king's power. People who survived the war got put to work and had more rights/power, shifting it away from the Monarch.