mechanically, it's two things: 1) the share of non-white voters that are black is decreasing, so the share of non-white voters that votes republican goes up (but shor says from 2012 to 2020, this effect is very small), and 2) in 2020 the electorate repolarized a little more on education and a little less on race/ethnicity.
leaving aside the why, that's basically the how. no college non-white voters voted for republicans a bit more and college white voters voted for them a bit less.