wakelaw, that's not a question that requires a 5 paragraph response. It's one or the other. You posted the question, "Why did the professors notice what they noticed?" You're saying two professors "noticed" Black students consistently get lower grades on an assignment they grade. That's like saying someone noticed a dead body while holding a smoking gun.
You mention affirmative action presumably as a rationale for saying Black students are systematically worse than White students. A university doesn't want professors who perpetuate stereotype threat by making comments like the one in the video.
I don't share too much about my personal life, but I have been a Black student or professor my whole life. I know a little about education. It's not like someone comes out and says "I gave you a lower grade because you're Black." White people seem to think that's how it works. It's not. It's seeing white people get opportunities and privileges you didn't know existed. That's why this video is a gut punch to Black students. Imagine being an Black alum and finding out that the grade you got in Due Process is the grade they've been giving to Black students this whole time.
Are you arguing that the two professors didn't give the grades? There was an investigation that consisted of talking to both professors. I guess it was short and conclusive and they made their decision.