Why did the barons turn against de Montfort?

Simplistically put, the Barons' War was fought over money and power; the major nobles of England thought that King Henry III had too much of the latter and was exercising it poorly. Henry needed more money for his wars against Wales and France, and to support a papal crusade. ... This stirred his barons to action.

Explanation:

The Second Barons' War (1264–1267) was a civil war in England between the forces of a number of barons led by Simon de Montfort against the royalist forces of King Henry III, led initially by the king himself and later by his son, the future King Edward I. The barons sought to force the king to rule with a council of barons rather than through his favourites.