I also think lately Google has been giving a bump to fresh content. If this bump is what has been keeping your site on the front page, and Googlebot has gotten used to seeing new content being added regularly, changes in that schedule could cause you to lose positions - especially if you lose the freshness bump. I see this mentioned more in relation to newer sites, but I think it can be an important factor on established sites as well. I also suspect that this may help the overall site more than the individual pages (possibly following the internal link structure) so that Google can highlight pages that are frequently changing and may be of more benefit to searchers.

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