I think that a lot of people are confusing correlation with causation.
In reality, the most plausible explanation is that a number of different (but often related) events combined to a give "network effect". In other words there is certainly no single cause, and quite likely no major cause.
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 answers attempted to draw relationships with events much earlier; e.g. Linus Torsvalds evangelism for DVCS, formation of Github. While there is no strong evidence of correlation (on a time line), that does not exclude causation ... if you accept the "network effect" hypothesis; i.e. that there are multiple causes.
IMO, the "network effect" explanation