The term for this is intrusive thoughts. I often have those about my own death. Some I've become numb to, like car crashes for example. Others can be super shocking. I can't think of any of those currently because it's usually a new scenario that my mind hasn't considered. My understanding is that the brain creates and plays out these scenes in a way to prepare itself in the case that it happens. Sometimes the intrusive thoughts are random. Those of us with PTSD are far more prone to intrusive thoughts because of hypervigilence. Our bodies and our brains know first hand that bad things can and do happen. This is our brains way of preparing for any scenario that may happen to us.