What factors accounted for immigration becoming a global phenomenon during the late nineteeth century?after the civil war many southereners...

You seem to have two questions, one in the header and a separate one after.  I'll concentrate on the first one.

A number of global factors contributed to widespread immigration, much of it to the United States.  These included overpopulation in China coupled with civil unrest and the Opium Wars.  There were wars in Europe for the unification of Germany and between Germany and France.  Poverty and hunger were widespread in Ireland for much of that century.

It also became cheaper during this time to travel by boat across the Atlantic Ocean.

Add to all of this there was a lot of available land in the US, which attracted landless farmers from Europe, along with the economic opportunity (or the illusion of it) of factory work in the cities.  Many Chinese male immigrants came here for the gold rush and stayed to work on the transcontinental railroad.