It seems that BMW has been using a technique known as a “doorway page” on its bmw.de homepage. The technique, used by less-than-ethical websites, provides a keyword-rich page for search engine web bots such as Google's. But the page is only for the ‘bots; flesh-and-blood visitors are directed to another page.
Google, which posts policies against such deceptive practices, reacted swiftly once it received the news and has put BMW under what is known as the “Google death penalty”. The article provides more information on both the penalty and the incident.
Although BMW has removed the offending pages, there has been no official explanation released by the automaker.
[Thanks for the tip, starlightmica.]