In the NBER reporter, Allison Shertzer and Randall P. Walsh summarize some of the recent research on urban segregation. As a cause of early 20th century segregation, Shertzer and Walsh put somewhat more weight on white flight and local land use regulations and somewhat less on redlining and discrimination by the Federal government (although all causes were important).
One point which I had not previously considered is that technology interacted in important but unintended ways with preferences for segregation:
We hypothesize that public transportation was critical for the acceleration of white flight because streetcars and subways significantly reduced the cost of living further away from employment centers. Household preferences for racial composition could have interacted with municipal infrastructure investments to increase residential segregation. Such a finding would further underscore the lesson that policies that were race-neutral on their face likely contributed to the development of segregated cities.