BuddyPress.org

Cannot pinpoint the exact reason but it’s most likey the IE box model bug http://en.wikipedia.org/wiki/Internet_Explorer_box_model_bug and the “quirks mode”. You can read the article and go to links referenced to get a better grip. Web Standards/CSS guru Eric Meyers and many others have expanded on that issue among many other IE sins since way back.

I saw a WP plugin which renders a pop-up or a warning after “sniffing” online visitor’s browser, to upgrade to latest browser version, you could search in WP repo. There’s also a way to do this without a plugin and you can search online for something like “add script warning upgrade to latest IE version”.