It is in part due to middle-class, upper-middle class, and wealthy African peoples moving to the US.
Many of the peoples immigrating fall into the aforementioned classes, many more also come with undergraduate or graduate education; just as has been shown with many of the East Asian immigrants of the last 30-40 years, even if one was not wealthy in their own country, their previous education is a strong bridge to US educational resources and subsequent stable and high-paying careers.
People aren't leaving their country and immigrating to the US without connections, education, or exceptional drive, the third of which has a very common presence in immigrant and migrant US communities (including Southern African-Americans who moved North or West during the Great Migration.)