There isn't an easy or simple answer to the question of complex regional pain syndrome (CRPS). Scientists are conducting many studies to sort out all the variables and factors that go into a condition like this. So far there isn't agreement about the mechanism of cause.
It may be that nerve damage after an accident or injury occurs setting off this extreme response. Some doctors and scientists think the central mechanisms of the nervous system are triggered by the injury. Their signals get mixed up and reorganized in a chaotic way.
For now it seems that there' isn't a single one-way to explain what went wrong. Once the underlying pathology is discovered treatment will be able to address the cause instead of just the effects (symptoms).