When towns started to grow more trade was happening which meant harder for the king to control or have power which leads one reason to decline of feudalism
By helping shift power from feudal lords to the monarch and common people and Also, changes in military technology reduced the need for nobles' knights and castles. This is how the 100 years of war lead to the decline of feudalism
More people were dying from the plague so there was less people to do work and them the feudalism system was coming down.
The Magna Carta was forced to be signed by King John and it showed that people had more rights which meant there was not a need for the feudalism system