Why did the pope excommunicate Charles-Maurice de Talleyrand?

Charles-Maurice de Talleyrand was excommunicated by the Pope primarily due to his instrumental role in the Civil Constitution of the Clergy, which reorganized the French church on a democratic basis without papal approval.1

As a deputy during the French Revolution, Talleyrand further alienated the church by urging the repeal of the tithe and advocating for the nationalization of church property to address state debts. His actions culminated in him becoming the first bishop to take an oath of loyalty to the new constitution and consecrating the first bishops elected under its procedures. These revolutionary activities led to his excommunication, though he remained largely indifferent to the censure as he was already planning to depart from his ecclesiastical career.1