|The ruins of Kaiserswerth in Dusseldorf|
It may have been a simple "power grab" by men who wanted to run the kingdom themselves, or by men who wanted to save the kingdom from Agnes (she had given away the duchies of Bavaria, Carinthia, and Swabia). It may also be that the conspirators felt the kingdom should not be ruled by a woman.
Bruno the Saxon, an 11th century monk who wrote the Historia de Bello Saxonico ["History of the Saxon Wars"] claimed that Henry's behavior prompted the drastic action, because he was arrogant and would not listen to his mother. Archbishop Anno did the right thing by taking control of Henry.
For the Coup, Anno invited Agnes and Henry to stay at the palace of Kaiserswerth on the River Rhine in Dusseldorf. After dinner, the archbishop invited young Henry to see his fancy new boat. Once onboard, the boat cast off from shore. Exactly what Anno's plan was is not clear, but Henry feared for his life and jumped into the river (putting himself in far more danger than staying on the ship). One of the nobles present, Count Egbert, dove in and saved Henry. The ship was rowed to Cologne, where Henry was held until Agnes agreed to surrender the regalia.
Agnes went into a convent and Archbishop Anno became regent, ruling the country until March of 1065, when the 15-year-old Henry was crowned. This sounds like a happy ending for Henry IV, but his reign would be troubled by many issues and incidents, including the Investiture Controversy.