My understanding is that the causes of hair pulling are not known. One thing that does seem to be the case, though, is that stress can aggravate the condition. Here are links to some articles, from PsychCentral's archives, on the subject of trichotillomania:

https://blogs.psychcentral.com/menta...chotillomania/

https://psychcentral.com/lib/an-unco...chotillomania/

https://psychcentral.com/lib/tips-th...dium=popular17

https://psychcentral.com/blog/my-lif...-hair-pulling/