With the new Bigdaddy update, crawling is changing. A forum regular posted a problem on this website, where pages were dropping out of the index. Other users have had the same problem.

GoogleGuy replied:

One thing to bear in mind is that Bigdaddy will have different crawl priorities. That can account for some of it. If you’ve run into any spam problems in the past, you might also want to do a reinclusion request. Otherwise, please send an email to bostonpubcon2006 at gmail.com with the subject line “crawlpages” (all one word), and I’ll ask someone to see if they notice any commonalities.

What factors determine the crawl priorities? Up to now, we don’t have much information yet but it is interesting to see how webmasters can get their pages back on the index.

Sources: WebmasterWorld, (2).