That is the big question. We know Huber and Gaige attacked Kyle because they thought he murdered Rosenbaum. Somewhat reasonable. Rosenbaum is the odd one.
Did Rosenbaum aggress Kyle simply because Kyle was alone, as he threatened to do earlier in the night? Was it because Kyle had a weapon and Rosenbaum took issue with that? Was it due to Kyle putting out fires that Rosenbaum may or may not have started? Rosenbaum's mental state was called into question, so did his medication (or theoretical lack of medication) play a part? Only Rosenbaum or Ziminski would be able to answer these questions.