Sure there is…you just don’t know what it is.
Now, why did the emperor call another council? Well, first we have to understand which emperor, but a council was ultimately called because, as was often the case, heresy was spinning out of control in in the East.
These events have been documented by others, so I’ll just rely on their words:
In the 440s, a respected monk from Constantinople, Eutyches, denied that Jesus was truly human. He taught that Jesus did not exist in two natures because his human nature was absorbed or swallowed up by his divine nature. Flavian, bishop of Constantinople, convened a synod in 448, condemning Eutyches’ position, but Eutyches appealed the decision. The fight took a nasty turn when Dioscorus, Patriarch of Alexandria, became determined to reinstate Eutyches and his views. Eastern emperor Theodosius II, also favoring Eutyches’ position, called another church-wide council to meet at Ephesus in August 449. He appointed Dioscorus to chair the proceedings and to silence any dissent.
Leo I, bishop of Rome, sent delegates to the synod with his Tome, an exposition of how the two natures, divine and human, are joined in Christ. Dioscorus prevented the reading of Leo’s letter and rejected his position. Eutyches’ teaching was declared orthodox. Bishops who refused to accept the council’s decision were deposed.
Council of Chalcedon
An unexpected event dramatically changed the situation. On July 28, 450, while out riding, Theodosius’ horse bolted. The emperor fell, broke his neck and died. His sister Pulcheria became empress with her husband, Marcian, as co-emperor. They were opposed to Eutyches’ teaching and eager to redress the wrongs perpetrated by Dioscorus.
Emperor Marcian called for a church council to meet at Chalcedon, on the outskirts of Constantinople. More than 500 bishops attended — the largest church council gathering to that time. All delegates were from the Eastern Church, except the few papal representatives from Rome and two from Africa. Deliberations lasted from October 8 to November 1, 451.
Leo again sent representatives with his Tome, which was read and approved by the council. Chalcedon reversed the “Robbers’ Council” decision and condemned Eutyches’ teaching. It anathematized those who taught that Christ had only a single, divine nature and those “who imagine a mixture or confusion between the two natures of Christ.”
So, why was a council called to settle a matter previously addressed by the pope? Well, the pope had rejected Ephesus II, as did Chalcedon, but Chalcedon went further by condemning Eutyches, as well.
Isn’t Google
amazing!