I had a college professor change my first grade in the class from a B to a D and give me a lecture about “subjective versus objective grading”, after I asked hi

I had a college professor change my first grade in the class from a B to a D and give me a lecture about “subjective versus objective grading”, after I asked him for feedback. Apparently my B was supposed to be one of those and the D the other one. His rant didn’t have anything to do with subjectivity or objectivity so I’m not certain which was supposed to be which, but he acted like giving me the B was doing me a favor. This guy clearly grade-grubbed through school and assumed everyone else is just there for the grade too. I just wanted some feedback other than “B”, so I could, you know, learn.