I have a suspicion what you'll find is that Bing use the toolbar to match $current_page_content with $clicked_page_content. When $current_page_content contains obscure words, that becomes the only signal, and so bing's engine will naturally associate it with $clicked_page.

In other words, there's a relationship between Page A and Page B if there exists a link beween them (==PageRank). But the strength of the relationship is increased based on how many users click on that link. I think that's the information Bing were trying to capture (or if they weren't, they should have been).

What I'm saying is that it's probably an unintentional side-effect. At scale though, the effect is that Bing gradually uses Google as a signal, simply because Google is a popular site.

edit: Yet another way of saying it: I think it's not just clicks on Google searches that are captured by Bing, but clicks anywhere. Google is a large site, so its influence on Bing can be measured. This is what we're seeing. My theory. I don't work in search.