It's something you said to moon that triggers her death. When you talk with her in the kitchen or somewhere sitting don't exactly remember where I remember when Kei was sitting with her later in the game before the fight.
Or people are playing the game blindly not knowing the outcome of choices. I didn't know she could die, and i was taking my time reading all of the dialogue. I’m assuming it depends on whether you push your beliefs on Moon or not, which i did and Moon refused to kill her when Aoife asked
