The dispute that produced the excommunications of 1054 was, on its face, about bread. The Latin churches used unleavened bread in the eucharist and the Greek churches used leavened, and Cerularius regarded the Latin practice as a Judaising error that made the sacrament unfit. He closed the Latin churches in Constantinople, and a letter was circulated attacking western usage on that and other points.
Leo IX sent a legation headed by Cardinal Humbert of Silva Candida, a man of comparable temper, to settle the matter. The negotiations went badly from the start, Cerularius refused to meet the legates on terms they would accept, and on 16 July 1054 Humbert and his colleagues walked into Hagia Sophia during the liturgy and laid a bull of excommunication on the altar. A synod under Cerularius excommunicated the legates in return.
Two details matter for what the event actually was. Leo IX had died in April, three months earlier, so the legates were acting on the authority of a dead pope and their commission had arguably lapsed. And the documents excommunicated named individuals, not churches: Humbert's bull anathematised Cerularius and his supporters, and the Greek synod anathematised the legates. Neither side purported to cut off the other communion as a whole, and neither had the standing to do so.