http://www.straightdope.com/columns/read/2877/did-soldiers-really-frag-officers-in-vietnam
As Cecil points out, we have an absurdly small amount of data and no guarantee of its randomness, so this is really guessing and nowise conclusive. Still, it looks like a serious effort, so it's worth looking at if you're a fan of the fragging --> volunteer army argument.
And mind you, that argument (like the oil one) can be true causally even if it's not true factually. Like Hamid Dabashi said in his interview with Doug (paraphrasing Durkheim), If everyone believes something's true, it becomes a social fact that has effects. And Cecil gives no reason to disbelieve that army believing the fragging argument was a major contributor to the initial push towards an "all-volunteer" force.
I put "all volunteer" in quotes because it's one of the two greatest euphemisms the 20th century military establishment ever pushed through. Renaming the War department the Defense department was a great one. But renaming what had prevously always been called a professional army as the all-volunteer army might have been even more important.
Michael