Who's buying the stock?

There is growing evidence that more and more of the stock purchased in the markets (including the dark pools) is being bought as part of a corporate buy-back. Last year corporations bought $754.8 billion of their own shares. From 2006 through 2013, corporations authorized $4.14 trillion in buybacks of their own publicly traded stock in the U.S. Is this being done to prop up the stock or boost executive compensation or for some other reason?

Many corporations are resorting to borrowing in order to buy their shares. It does not sound good.