A few days ago, I've been facing a strange behavior with Visual Studio 2013.
No matter what solution I was opening, Visual Studio kept switching the source control plugin to Git. I was sure all my solutions were bound to TFS.
I didn't understand why Visual Studio was insisting. And when looking for a solution on stackoverflow, every single post were talking about switching or disabling the source control plugin in Tools > Options > Source Control > Plug-in Selection, or saying it was due to the hidden .git folder in the solution folder.
I finally found that it was my fault (who said of course? :-)).
The solution was located in :
C:\Users\Renaud\Documents\TFS\MyOrganisation\MySolution\
From: http://www.renauddumont.be/visual-studio-2013-always-switches-source-control-plugin-to-git-and-disconnect-tfs/