I always attributed it to light bleaching . Thinking back now I only remember it happening with one strain , Apollo 13 . At the time I was running it with Apollo 11 and SSH and didn't see it on those .
Just a thought . Another thought is that when I saw it the clones I was running were going on 2 years old , I wonder if age may make a plant more susceptible ?
In my cases it remained very localized and I ran the to the end without a problem .