Agree with jerf. And furthermore, looking at the chart provided its not clear to me that one is trailing the other. For example, Bing has a big dip on March 14th, which is prior to the date that Google does. And even before the March 15th date your incoming data isn't very flat. And during the March 7th week Bing seems to again lead Google on a downward trend.

I don't think you can assume anything from this data at all.

What would be far more useful, to prove this hypothesis, is request to Google to not show the site for a week and see if Bing's results fall off a cliff. I suspect they won't, but give it a shot.