The short answer is we don't know.

Fibro is one of a number of disorders called functional disorders that are classified by their symptoms rather than their cause, because the cause isn't known or is disputed.

Other common function disorders you have probably heard of are chronic fatigue syndrome (aka ME) and irritable bowel syndrome. But there are loads others as well. Eg non epileptic seizures, tremors, blackouts, walking disorders, and some really weird stuff like not being able to move certain muscles.

Source I have a walking disorder, cfs and some fibro.