IMO A lot of that decline/increase is probably where Bing has bought integration into an existing product. I've seen it a lot lately, "powered by Bing" and the like, but I've not seen a single person actually type Bing into the address bar. It seems to me that this increase is coming from those people who don't know/care and are just being directed to Bing, not making a concious choice of which search engine to use.