It appears that Matt Cutts of Google has added a tidbit to his already mind-swimming post named Scoble visiting the Plex, Matt added;
By the way, it looks like the primary issue with the Windows Live Writer blog was the large-scale migration from spaces.msn.com to spaces.live.com about a month ago. We saw so many urls suddenly showing up on spaces.live.com that it triggered a flag in our system which requires more trust in individual urls in order for them to rank (this is despite the crawl guys trying to increase our hostload thresholds and taking similar measures to make the migration go smoothly for Spaces). We cleared that flag, and things look much better now.
A sudden increase of URLs on spaces.live.com triggered a Google flag, once they cleared "that flag," things began to "look much better."
So adding new URLs or Sub domains can trigger such a flag? Google has flags that bring up manual review?
Sites with seasonal products that add hundreds of products during that season can be affected? I doubt it. Revamping a site's URL structure can trigger a flag? Depends on if this is a subdomain thing or not...
There are just now so many questions an SEO may have from this post.
Forum discussion at WebmasterWorld.