Dymero - 11:41 pm on Feb 25, 2013 (gmt 0)
I think goodroi is right. I occasionally look at some keywords over at Google Trends, to check if they're still relevant. For some of those, you can quite clearly see the trend that they're becoming less popular.
Also, I find that Google sometimes demotes pages for certain keywords without rhyme or reason. For example, a page that was doing extremely well for a keyword early last year suddenly dropped like a stone for a few months, and then returned again without explanation. I made no changes to the page during that time. Simply no explanation.