it's difficult to rule anything completely out when no one knows the cause of fasciation. I wouldn't even go as far as to make a binding statement that it is non-pathological (not cause by a living organism), although I would say the odds are extremely minimal. Nature is incredibly diverse in all its forms (including diseases).
I could see a virus, mycoplasma, or whatever being triggered into action by some external impetus. What makes us get cancer, why can it go into and out of remission, what makes someone who had chicken pox decades ago suddenly get shingles... or never get shingle, for examples.
That certain individual plants seem prone to fasciation would tend to support a pathology, or tend to not support a pathology, depending on how you look at it.
