The same post-war migrations that saw many black Americans move northward also saw a smaller but still significant population move to westward, especially to port cities like Los Angeles, San Francisco, and Seattle. This was mainly for the Second Great Migration after WW2, not the first one in the 1920s.

A large part of this shift began with World War 2. Many African-Americans joined the military and were often assigned support roles, landing them jobs working around naval shipyards for the Pacific theater. After the war ended there was still a huge demand for those kinds of industrial jobs. The west coast's manufacturing sector was much more developed by this time than it was in the 1920s.

So between this huge demand for jobs and a sizable population of ex-military black Americans staying behind, there was a migration boom. Ex-soldiers would bring their families from the South to live with them. And they'd also spread the word about this demand for jobs and the existing black community on the west coast, which would draw in friends and siblings and cousins, and all of their families as well.