I imagine the best thing would be to just be honest? You obviously passed the interviews and thus are qualified enough. Maybe the coursework wasn't very engagin

I imagine the best thing would be to just be honest? You obviously passed the interviews and thus are qualified enough. Maybe the coursework wasn't very engaging or a lot of "empty work" was required and you decided to focus on side-projects instead of chasing the high GPA. Or you decided to work on the side? Whatever it is, I believe the explanation would make sense.