tedster - 5:34 am on Nov 15, 2012 (gmt 0)
Gieven what we see reported here, most crawl spikes have to do with Google's technical scheduling needs for large groups of sites, rather than being specific for just one. Because of this, I'd be on the lookout for comment from others that they also saw a crawling spike at the same time. The exact days of the crawl spike might help.
If you recently made some changes to your site structure, or to the content of many changes, that might be one reason Google needs a more widespread crawl of your particular domain. However, even a crawl like that might be scheduled along with deep crawls of other sites.