It's not clear that they are doing that. Web logs I've seen from other writing on this topic show them re-crawling the same pages at high rates, in addition to crawling new pages
Actually I've been informed otherwise, they crawl known links first according to this person:
> Unfortunately, based on what I'm seeing in my logs, I do need the bot detection. The crawlers that visit me, have a list of URLs to crawl, they do not immediately visit newly discovered URLs, so it would take a very, very long time to fill their queue. I don't want to give them that much time.