Hey y'all. So George Lucas gets a lot of crap for ruining the Star Wars franchise. The original trilogy he created back in the day was a fun romp through a science fiction world, and a great classic adventure. And then he created the prequels, which ended up being everything the originals were not: boring, overly digital, unexciting, and having a plot so complicated no one's really sure what happened. No one cares, that's for sure.
So what happened? Did Lucas' head get too big? Did he care more about the money than doing his job? Is he really a terrible storyteller who needed lots of help from other people to create the original trilogy? Well, I would like to submit that George Lucas is an extreme example of what can happen to potentially any writer. Hence, George Lucas disease.
But wait, you protest, how dare I ascribe to all writers the possibility of Lucas' decrescendo? Lucas obviously lost his touch, and that doesn't happen to everyone. Yes, it doesn't. Not to that extreme. It is, however, something that can happen when a writer works too much on one franchise or that franchise becomes extremely popular. Chances are, a writer won't fall nearly as hard as Lucas, but keep in mind that Lucas had a much higher cliff to fall from; Star Wars is a franchise with lots of expectations on it. Few writers reach that level.
So what is George Lucas disease? It's the swelling of a story franchise to the point where people are sick of it, and the writer is incapable going interesting new directions. There are many causes to it, but the results are all the same; people (besides uber fans) stop caring.