Why did Philip II of France declare King John’s French fiefs forfeited?

Philip II of France declared King John’s French fiefs forfeited in April 1202 primarily because John failed to appear before the French royal court to answer for his actions in Poitou.12

The conflict originated when John married Isabella of Angoulême in August 1200. Isabella had been betrothed to Hugh IX de Lusignan, a member of a powerful Poitevin family. This marriage provoked a rebellion by the Lusignans, who subsequently appealed to Philip II, their overlord, for justice. As the king of England held his French territories as fiefs from the French crown, Philip summoned John to his court to address the grievances of his vassals. John’s refusal to present himself provided Philip with the legal justification to pronounce the forfeiture of John's French lands and to personally enforce that sentence through military conquest.12