I'm surprised by the decline of C#. I thought it was better off than java these days. Also PHP is gliding down. They improved the language a lot, but I can't say I will miss the PHP world much.
The graphs are for # of views to questions on SO involving that language. What it likely means for C# (and PHP/Java, etc) is that the user-base is maturing and stabilizing, and not as many questions need to be asked (or found). For C#, it could also have something to do with the fact that the MSDN .NET docs have vastly improved over the past 3-5 years.
About python, isn't it related to numpy mostly ?