adCenter announced that "they'll be temporarily increasing the standard crawl rate they use for detecting ad relevance on adCenter ads:
Along with our consistent efforts to increase relevance for your adCenter campaigns, we'll be temporarily increasing the rate at which your sites will be crawled to 20 to 30qps. This's within the accepted standards for crawl rates, and is vital to continuing to provide most relevant advertising experience possible. If you've any concerns with this increased rate, please contact your account management team as soon as possible.
Note: this crawl rate increase is only applicable to Microsoft AdCenter ad index crawler (adidxbot) and is crawling advertisers only. This doesn't apply to Bing index crawler (msnbot).