catholicherald.com/saunders/98ws/ws980423.htm
"…For the greater part of this century, the faithful recited the prayer to St. Michael at the end of the Mass.
Pope Leo XIII (d. 1903) had a prophetic vision of the coming century of sorrow and war. After celebrating Mass, the Holy Father was conferring with his cardinals. Suddenly, he fell to the floor. The cardinals immediately called for a doctor. No pulse was detected, and the Holy Father was feared dead. Just as suddenly, Pope Leo awoke and said, “What a horrible picture I was permitted to see!” In this vision, God gave Satan the choice of one century in which to do his worst work against the Church. The devil chose the twentieth century. So moved was the Holy Father from this vision that he composed the prayer to St. Michael the Archangel…Pope Leo ordered this prayer said at the conclusion of Mass in 1886. (When Pope Paul VI issued the *Novus Ordo *of the Mass in 1968, the prayer to St. Michael and reading of the “last gospel” at the end of the Mass were suppressed.)
In the Spring of 1994, our Holy Father, Pope John Paul II, urged the faithful to offer the prayer to St. Michael the Archangel. He also made the strong suggestion that the recitation of the prayer be instituted at Mass once again. (Note that the Holy Father did not mandate the recitation of the prayer at Mass.) Clearly, the Holy Father was responding to the grave evils we see present in our world — the sins of abortion, euthanasia, terrorism, genocide, and the like. As we approach the millennium, Satan and the other fallen angels are doing their best to lead souls to Hell. We need the help of St. Michael! For this reason, many parishes have erected a shrine in St. Michael’s honor or offer the prayer at the end of Mass or after the petitions."
Why after Communion, instead of at the end of Mass? I couldn’t tell you. Perhaps the pastor feels that there is such a “Mass Exodus” at the end that he’d better do it while everyone is still “prayerful.”