Like with any phobia one may never know.
However more common is a phobia caused by a reaction of someone or a learned pattern from someone. Or a phobia could be caused by a trauma they may not remember. This is the scenario I would assume was the case of this phobia. An irrational fear of irregular patterns of holes or bumps sounds like a scenery one may have seen while a trauma took place. The brain is a remarkable organ to say the least.
This is just my uneducated guess. :0)
