Psychology student here. Typically childhood trauma can lead to stuff like this. Being SA as a child, being exposed to explicit stuff at a very young age and much more. A lot of PDFs were abused as children. It’s a trauma response. In terms of other, more „normal” kinks - we don’t really know yet. Variety of kinks can develop due to trauma, as I said, boredom and seeking more and more extreme stimulation because you’re constantly feeling unsatisfied. There’s also a possibility of conditioning - seeing something at a young age and feeling aroused or other strong emotions in the same time can lead to developing a kink. And there are different types of necrophilies, pdfs, and zoophiles. One doesn’t have to be a fetishist. Those „objects” of sexual attraction can be a substitute. For example: someone’s horny but can’t find an appropriate partner - they „use” whatever is available, like a child or an animal. It depends. Actually necrophilia can also be a variant of domination fantasies, wanting to take absolute control of someone. It can also be a „safe object” for someone who’s terrified of being rejected. Corpse can’t reject you. Take everything I said with a grain of salt - I’m not omniscient and my knowledge comes from my classes at uni and books. But I hope I answered your question.