Not a failure of willpower
BFRBs can happen automatically, or with an urge that is hard to resist.
BFRBs are repetitive, body-focused behaviours such as hair pulling (trichotillomania), skin picking (dermatillomania, sometimes called excoriation disorder), nail biting, lip biting and cheek biting. For some people they happen almost entirely outside awareness, while reading, watching television or concentrating on something else. For others there is a build-up of tension or urge beforehand, a sense of relief during or afterwards, and sometimes a wave of shame or frustration once it isn'ticed.
Whichever form it takes, a BFRB is not simply a failure of willpower or self-control. It is a genuine, learned pattern that the nervous system has come to rely on, in much the same way as other automatic habits.
Possible triggers vary from person to person, and may include tension, anxiety, boredom, particular sensory experiences, or a pattern that has simply become automatic over time through repetition. Not everyone experiences the same trigger, and it is entirely normal not to know exactly why the pattern started.