I'm not rationalizing it, I'm describing why perception has changed.
I you're a 'white guy' working in NYC in 1960, it's easy to see how desegregation is 'good thing' that doesn't challenge your status.
If you're a 'white guy' in Alabama in 2016, and know that 4% of the spots in your College have to go to 'people of colour' - in 2016, 50 years after desegregation wherein most people have at least some opportunity - you might be more likely to perceive that specific scenario as zero-sum.
I you're a 'white guy' working in NYC in 1960, it's easy to see how desegregation is 'good thing' that doesn't challenge your status.
If you're a 'white guy' in Alabama in 2016, and know that 4% of the spots in your College have to go to 'people of colour' - in 2016, 50 years after desegregation wherein most people have at least some opportunity - you might be more likely to perceive that specific scenario as zero-sum.
It's perception, not necessarily reality.