Hard to establish if any one thing is the catalyst.  And it's equally hard to say if relations between different races, religions or socioeconomic groups were demonstrably better 30, 50, or 100 years ago.  I do know that a lot of extrajudicial justice (lynching) was dispensed in our past, some victims being white as well as the better documented cases involving people of color.

As a general position I'll broadly say that people tend to get along better if their shared society is enjoying a time of peace, and if resources and opportunities in the form of stable employment, affordable housing and other material middle-class creature comforts remain realistically obtainable by citizens of average means.  I do think if a society is undergoing considerable stress such as being involved in a major war or a prolonged and obvious disparity between its rich and poor, then the tendency or excuse to seek out villains, real or imagined, as well as hapless scapegoats becomes hard to resist.