SERP Drop!! What could be the reason?

I am just as confused as you. Usually when SERP declines, everything else does. Not always, but sometimes. Maybe you're getting more traffic for a particular term somewhere.

There will always be movements in the SERPs! And the variables that cause the movement could be but not limited to:

(a). Direct competition adding/building BLs
(b). A flood of new entries of articles, videos, blog posts, etc.
(c). SERP Algo changes
(d). Temporary impact from improper backlink building tactics
(e). New websites launched
(f). Particular keyword phrase made major or mid level news syndication

There are other variables but those are some of the top reasons.

What you experienced is normal. The key to a continued rise to the top and then grabbing and maintaining that top position is simple; keep building BLs and diversify with a mixture of high PR and low PR BLs and a percentage of no follows so your link building looks natural.

Once you get to the top spot, don't stop! Push enough BLs so the average IMer will analyze your site stats and go, "It's not worth it to try and knock him off...its just too much effort and too much work!" ROTFLMAO

The reason this is so funny is because most IMers have been brain washed not to go after keyword phrases when they see a certain amount of BLs and PR.

Giles, the Crew Chief