What factor accounted for immigration becoming a global phenomenon during the late nineteenth century?

I'd say there
were factors on both sides of the Atlantic.  Inthe United States, sudden and rapid
industrialization during the Gilded Age created a huge demand for labor at the same time as the
Western territories were being secured and cheap or free land became available.  This was an
irresistible magnet to people on in Europe and Asia as they sought to escape poverty and chaos
in their own nations.

In Europe, economic issues and political upheaval,
especially in Germany and France during that time, led to large numbers of germans, Poles,
Russians, Jews and Italians immigrating in the same 30 year time span.