A category page on our site disappeared during the last week of September. For about 3 years it was near the top of page 1 for a variety of competitive terms. No changes had been done to the page in about a year, except product additions.
I did a site: search and took a snippet of text from the page; we are no longer showing in the index. A product page from within the category is actually showing on page 2 for the same terms now. I checked Webmaster Tools and it doesn't show any problems and still shows it in the Sitemap. It still has Pagerank. It still ranks well in MSN and Yahoo.
Using CopyScape, I did find some scraper sites had copied a lot of the text from the page; they are actually showing up in the Google results when I copy a snippet of text. I went ahead and "100%-ly" changed the main content of the page.
Any ideas why this might be happening? We haven't built a lot of links directly to this page in quite a few months, but I would think it would have been penalized on a keyword level instead.
I would think it would have been penalized on a keyword level
What do your logs tell you? If the page now gets no entry traffic from Google Search at all, that sounds like a page level flag. If it does get some search traffic but not for the important keywords that used to be working, then yes, it's a keyword filter.
I think I see Google tweaking the anchor text area of the algo. At least I've seen several examples where rankings were lost on a keyword when that word was "unnaturally" featured in backlink anchor text.
Yes, it sounds like your page is filtered because Google lost track of which URL is the original version. What a pity that you changed your content because you were copied. I'd rather go after the copier with some DMCA sauce. After all YOUR original page was ranking.