From The Daily:
University Police Lieutenant Robert Neumann said the victim was picked up and “body-slammed” to the ground by a man, who was accompanied by two other men. Neumann said police have a suspect, whom he described as a 22-year-old male University student from Ohio.The suspect mentioned is Mike Milano, a walk-on on the University of Michigan football team. This is mildly-educated speculation on my part, but I've got a feeling the other two people with Milano were football players as well.
Obviously there's very little context to these events, but it sounds like Kampfer wasn't at fault for what transpired. According to Red Berenson:
“I can’t tell you that he did anything bad,” Berenson said. “He just was a victim.”