In June 1170, the Archbishop of York, the Bishop of London, and the Bishop of Salisbury, crowned the heir apparent, Henry the Young King, at York. This was a breach of Canterbury's privilege of coronation, and Becket excommunicated all three.
Upon hearing reports of Becket's actions, Henry is said to have uttered words that were interpreted by his men as wishing Becket killed. The king's exact words are in doubt and the most commonly quoted is "Who will rid me of this troublesome priest?" Or perhaps "What miserable drones and traitors have I nourished and brought up in my household, who let their lord be treated with such shameful contempt by a low-born cleric?"
Whatever Henry said, it was interpreted as a royal command, and four Knights set out to confront the Archbishop. They arrived at Canterbury and informed Becket he was to go to Winchester to give an account of his actions, but Becket refused. It was not until Becket refused their demands to submit to the king's will that they retrieved their weapons and rushed back inside for the killing.