I think that a lot of people are confusing correlation with causation.
The graphs presented all show correlations between measures of gits popularity and events ... and other measures. However, a correlation is not clear evidence for causation.
Some other answers attempt to draw relationships with other things; e.g. Linus Torsvalds evangelism for DVCS, formation of Github, the rise of social networking. While there is evidence of correlation (on a time line) is not as strong, that does not exclude causation. Especially if you accept the "network effect" hypothesis; i.e. that there are multiple causes.
The bottom line, is that the kind of evidence that is available cannot show causation. We are talking about collective behaviour of hundreds of thousands of people, and people make decisions for different reasons ... or no logical reason at all. Programmers are no different from anyone else.