This is not just a Christian thing to do, but an aspect of globalization, too. Nigeria, an English-speaking country, has what economists call a comparative advantage when it comes to priest production. There's also the factor of religious hunger. While India and a few other nations produce greater numbers of priests, Nigeria probably exports more than any other country today. The Vatican doesn't compile statistics on priest exports by country, but the U.S. Conference of Bishops estimates that Nigerian priests are the most mobile.
Hundreds work in Catholic dioceses in the U.S., filling a growing number of vacancies.
Nigerians are also prominent in Britain and Germany, where vacancies have risen. For a century, the Irish played the role of Catholicism's reserve army. Now it is the Nigerians' turn. Increasingly, they are the public face of Catholicism. For some, this takes getting used to.