The sites who moved up in the rankings may have gotten quality links that boosted their relevancy (as opposed to you losing relevancy).
You may have lost quality links
The sites that leaped ahead of you made big improvements in their SEO and now are reaping the rewards
There was a small algorithm change that played to the other sites' strengths
There was a small algorithm change that played against your weaknesses
Also keep in mind that rankings naturally fluctuate. It's very possible you will return to your previous rankings soon without having to change anything on your site.
One thing you should do is check to see if any of the sites that moved ahead of you are new or if they made any changes (if you don't check out the competition regularly you may not be able to do this). If so, you may want to make similar changes and see if it helps you. Be prepared to roll the changes back if they do not help or hurt your rankings.
My advice whenever someone has this occur is always don't panic and don't change anything. This is a perfect opportunity for you to make changes that you really don't need to make and they end up hurting you in the end.
If your content is still strong then wait it out. Keep getting links to your pages. Don't panic. Be patient.
To diagnose a drop you need to:
Undo any recent changes and wait to see if your rankings revert back.
Make sure you are up to date on Google's latest changes to their algorithm and see if any of them apply to you. If so, make changes that should affect that algorithm change and wait to see what happens. If nothing changes then rollback your content as that was the last known well-ranking format for your content.
That's about it. If you didn't change anything and Google hasn't made any announced changes then you have to assume your competitors got better either through better content/SEO or minor tweaks at Google that were unannounced and difficult to test for. In this case just keep plugging away. If your content is strong, you will regain your old positioning at some point.