1. They aren't Americans.  They are illegals or refugees who are afraid to be sent back to their home countries.
2. They aren't afraid of Trump.  They are afraid because Trump's policy towards illegals has been vague and sloppily implemented.  If someone were to come on TV tomorrow and say "Refugees seeking asylum will NOT be deported" or make some other such clear statement, I'd expect you'll see that flow of bodies slow somewhat.