PTSD develops after someone has experienced or witnessed a trauma where there was an overwhelming risk of major harm or loss of life. The trauma experience can be so confusing that the brain desperately tries to make sense of it, but can't. The brain sometimes gets stuck in the search for why the trauma happened or makes a low resolution judgement about who is to blame. Sometimes no one is to blame, and it was just a random event, sometimes someone was definitely to blame because they were malevolent. Sometimes someone failed to act to prevent it, either knowingly or unknowingly.