Body Integrity Identity Disorder (BIID) refers to a neurological and psychological mental disorder implying a psychological feeling that one would be happier living life as an amputee and is usually, if not always, accompanied by the desire to amputate one or more healthy limbs in order to enact that desire.

I was wondering what is the cause of this disorder and if by any chance it would be related to a past life experience? Or is it just brain chemistry going wrong.

[ [link to en.wikipedia.org]

SOZT
Many mental states involve self mutilation of some kind, so this is not a single occurrence. They all have at their base either self hatred or fear. If self hatred, the thinking is that if one mutilates themselves, then those who might do it TO them will feel placated. Father is going to beat me, so I will beat myself ahead of time to avoid his punishment, is the thinking. Anorexia is a desire to avoid looking pregnant or being sexually mature, avoiding Mother’s anger at having a competitor for Father’s attentions. The thinking is, that one should starve the self, or throw up the food eaten, as a punishment as if the child is challenging Mother, then the child is bad and must be punished. Mutilation can be to deliver pain or to change the appearance, to be less beautiful or attractive. Removing limbs should be seen in this light. If one cannot move, can one be punished for what one might have intention of doing?
EOZT