Yes. Bing is ... um.. kind of "picky" about what they index. If you browse through the Bing webmaster tools forums, you can lean a lot about how they choose which pages to keep in their index. Here are a few highlights:
#1 Bing will remove a site from it's index if it finds too many erors in your site map (404 errors, etc...)
#2 Bing will remove a page from it's index if it doesn't rank high enough.
Bing will actually remove a page from it's index if there are (quote) "700 million pages that rank higher then you" . Apparently the thought behind this, is that Bing doesn't want to use space to save a site that isn't ranking yet anyway... Once you build some more links to your page try resubmitting your sitemap.
#3 Bing is always "testing". Bing wil actually move you around the serps to test how people react to you. Bing will mesure the bounce rate of your page, how long people stay on your page, and the general way that people react to your page. If your page doens't pass the "Bing tests" then they might remove you from their index.
So in summery, Bing is trying to keep their serps unique by making it harder to get included in them. They are testing user interaction and they make decisions based on those tests. I know you can try to re-submit your sitemap to Bing after you've made some changes... Best Of Luck.