You left out a pair of key facts.
How many physically distinct pages do you actually have?
How many ways can those pages be reached?
"Not selected" doesn't necessarily mean that g### didn't like the page. It also includes every page you've ever redirected, every non-significant parameter, you name it.
I don't see a drop in Pages Indexed, which should be the important one. I see a surge in Not Selected. So unless your site really got a lot fatter around the end of September, I'd look for duplicates. Forums are a good place to start, because a single page can easily rack up 10, 20, 30 and more URLs.