I've watched an interview with someone who has multiple personality disorder and it was really fascinating. She didn't tell what happened, but she must have had a really fucked up childhood, her whole arms were covered in self harm cut marks.

My understanding is that as a defense mechanism the personality splits into two (or more) parts and one takes the suffering so the others can live an otherwise normal life. So anytime she was abused the same personality would be in charge, making sure that the other ones do not have to experience that trauma.

There is probably a lot of complicated science too.