My daughter started doing this about six months ago. It originated from a small scab on her scalp. She started picking that and then it progressed to pulling single strands. We have it under control for the moment. We got her to wear a kerchief for about 6 weeks so that it acted as a barrier. We also became aware that it was very situational ie sitting in class and having to listen or watching Tv. So at home we made her wear the kerchief or a hat if she was watching tv. I also changed the part in her hair so that when her fingers went searching for the bald spot, it wouldn’t be there. Ie put an elastic on opposite side.
Anyway, we have it under control for the moment and just pray that it stays that way 🙂
Good luck, I know how distressing it is to watch