Just to let you know, picking skin is not inherently a symptom of some deep hidden trauma. Sometimes brains do things a little randomly without there being deep psychological reasons behind it all, and that’s fine, if that’s the case.
Talk to your therapist about it, but don’t think you need to go hunting down some secret trauma, if you don’t feel traumatized or don’t remember experiencing trauma. Because sometimes the answer is just “brains do be like that though”