Tonearm - 10:03 am on Sep 5, 2012 (gmt 0)
I've been on the first page for my highest-traffic keyword for at least two years, and I've been ranked as high as #3. I just checked the rank for the first time in about a month, and now I'm on the 5th page.
I've been online for about 10 or so years and I've never done any sort of link-building besides a DMOZ link. A couple months ago I de-optimized my site to huge benefit.
How can I figure out what happened? The first thing that came to mind was a competitor pointing a bunch of dirty links my way. How can I check on that?