Subversion servers tended to hold more and more projects and branches and got slow on old hardware at least, as I experienced it, so I believe that inspired people to check out Mercurial and Git. However that on its own doesn't explain the explosion, as that would just show a transition of users from svn to hg or git.
Since the graph is of installation of git on Debian systems, part of the reason could be the explosion of Ubuntu use, but that was years prior, right?
The Linux kernel developers used git which provided more credibility. GitHub was a free hosting service and easier to use than Sourceforge and others, but that was years before also.
Michael's answer appears to be the best (need to add git-core and git use together), but I think that also Bundler and its support of git came around that time (late 2009):
http://yehudakatz.com/2009/11/03/using-the-new-gem-bundler-today/
RoR development doesn't account for the curve, but on the Rails side it seemed that this was when people started realizing it was much easier to have your own project and repo, that forking on GitHub started to make sense, and the word spread. That may account for only some of the acceleration around the time mentioned, though.