The Powerbook 1xx series has a few models that are very prone to this particular issue and it seems to be as Elfen and Uni say, either a weak spot when it was made that finally gave way due to age and use, or a small trauma to the screen that with age finally caused the scree to give, but either way, you have a screen that is not repairable. For some it seems to continue to spread (just a guess but I would imagine it has to do with the magnitude of the defect/damage that precipitated the problem), but for some it seems to stay contained. It's very difficult to predict which path it will take for a given screen. It's more of a wait/see situation. Your only hope is to replace the screen.