The reason for the bull was the Oxford Movement. There was a movement of Anglicans toward the Catholic church.
As the Oxford Movement
progressed, several members of the clergy and laity of the Church of England
argued that the Roman Catholic Church practice of unconditionally ordaining clerical converts from Anglicanism
arose out of a lack of inquiry into the validity of Anglican orders and from mistaken assumptions which, in the light of certain historical investigations, could no longer be asserted.
Those who were interested in a corporate reunion of Rome and Canterbury
thought that, as a condition to such reunion, Anglican orders might be accepted as valid by the Roman Catholic Church. A few Roman Catholic writers thought that there was at least room for doubt and joined with them in seeking a fresh inquiry into the question and an authoritative judgment from Pope Leo XIII
who permitted the question to be re-examined. https://en.wikipedia.org/wiki/Apostolicae_curae