The mass migrations during the Roman era occurred for much of the same reason now- exterior forces, war etc.

The germanic tribes of central europe were pushed by Huns invading from the east

Our southern border is being used by migrants to escape the economic and criminal issues in their homelands- both of which have been largely caused by US imperialism and economic domination for over 100 years- we installed dictators to do our military and economic bidding which failed to lead to stable societies