Angel, the course management system, calculates an overall grade in real time. This is good, but it means that as new components of the final grade come in, some grades adjust downwards. This generates a lot of email traffic. This is my generic explanation of what just happened.
I just released the attendance grade for the first time. Students have to be at nine pop quizzes to get this (worth 10% of final grade). We have now done nine, so regular attenders just got their 10%; those who have yet to be at nine just got a zero. They will get their 10% when they hit nine quizzes (and there will at least three more before end of semester). But for now, that means those who have missed any pop quizzes had their score go down by 10%. The scores of the other altered slightly as Angel re-weighted the various components of the grade to include the attendance score.