Catharism
In the South of France there was tremendous religious fervor, and an economy that was starting to grow, and a social class of  merchants and  peasants was starting to grow.  Peasants owned their own land.  Meanwhile, in other parts of Europe, peasants were forced to give up their land to nobles and become serfs or slaves - the system of feudalism.  There was a strong central absolute monarchy that did not exist in the South of France.  The burghers and bankers had more power in this looser system.  R. I. Moore is a historian who believes that it was desire to crush this system and take over the land that drove the attack.