From what I've seen people go from higher to lower rates all the time most likely because it's better than being vacant. It's worth nothing that most people probably know CM's rate was above average hence previous CM participants just went back to current average rates which I'm guessing has been followed by campaigns for some time already. 

The competition has also been increasing over the years, I would say posters who couldn't keep up has been weeded out over time as well. But since CM is a big campaign mostly filled with top posters plus it all happened in less than a week, it was more impactful/noticeable.