I was on my way to meet up with Rebecca for the first time, and I had five survivors with me. (Chad and his wife, LaShawndra - one of my favorite survivors - and her husband and I was carried Tracy, the old woman.) We met with Rebecca, and we were following her to the security room; since I knew the route (and there is the arrow), I moved ahead and guided the survivors to near the security room door with a way point arrow, when I realized that Rebecca was way, way behind us. I left the survivors at the door and went back to check on her. As we were returning, I saw that LaShawndra was standing next to the wall and good ole boy Chad was pointing his shotgun at her; there was not a single zombie around. Out of the blue, he just started shooting her until I triggered the cutscene.
I imagine it was just some glitch is Chad's AI where he was targeting a zombie through a wall or something, but given his accent and the way they were standing, it looked like it was the verge of some kind of racial execution.