Another thing that can happen, especially if you simply redirected the domain is that Google may have "lost" the site - it takes Google a while to find the new IP address of a domain, sometimes up to a week, because they cache known DNS entries. As a result, if your site was removed from the old host before Google got the new IP, Google might have knocked the site down because it thinks the site was inactive.

I have also seen a few comments in Google Groups about a possible sitemaps bug, that may cause a site to be dropped in the rankings when a sitemap is submitted for the first time. This is unconfirmed, but another possibility.

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The best way to learn anything, is to question everything.