The MSNBot that crawls the web for pages to include into the live.com search index is sometimes followed by another bot that behaves like a real browser, mimicing a real surfer.
According to what is known from Microsoft this is done to see whether the page is serving different content to search engines than it does to surfers (in order to manipulate search engine ranking).
In fact this is slightly stoopid, as the bot is easy to spot. Also the bot doesn’t honor robots.txt in every aspect, does download images, css and executes javascript and for some time even triggered adsense ads, screwing up the stats.
Also since the referrals are fake, it makes it seem like there is search traffic from live.com which in fact is not there. Last year the referrers had random search words, even porn an pharmacy related stuff. Now they try to use keywords that fit to your pages.
All pretty strange and noone seems to really know what’s the sense in that.
In essence: It is referrer spam from Microsoft.