Why did Thomas Becket excommunicate the clergy involved in Henry the Young King’s coronation?

Thomas Becket excommunicated the clergy involved in the 1170 coronation of Henry the Young King because the ceremony was a direct violation of both papal prohibition and the traditional, exclusive right of the archbishop of Canterbury to crown the English monarch.1

By having his son crowned by the archbishop of York—Becket’s long-standing rival—King Henry II bypassed Becket’s authority and defied the Roman Catholic Church. Becket, acting with papal authority, responded by suspending and excommunicating those who participated in the act, including the archbishop of York. This dispute over ecclesiastical authority and the king's interference in church matters remained a central point of contention, ultimately contributing to the tensions that led to Becket's assassination later that year.123