P.N.
I don't have personal expereince with this but the book I am reading addressed this behavior in toddlers. He likened it to nail biting, rather than being some sort of psychological issue (as it may be with an older child or teen). The 21 mo old child kept pulling out gobs of hair (she started at 18 mos).
The author (a child psychologist) told the parents she probably first did it accidentally but then continued doing it bc the parents started acting like it was the most important thing in the world.
He suggested they stop freaking out about it, be very matter of fact about it. When she would bring a hair gob, lead her to the bathroom and show her how to flush it down the toilet. Then walk away without saying another word. If gobs were found elsewhere, ask her to do the same.
In this case, he said the behavior stopped wihin a month.
So I can't personally speak to that. The book is by John Rosemond-Making the Terrible Twos Terrific. I agree with some of what he says generally, other things not, but he does seem to have a good idea of what is normal child behavior.