Share Buybacks via Debt
Based on the Fitch data referenced by Bloomberg, companies have been buying back shares over the last 3 years with more money than the free cash flow they earn (i.e. they had to issue debt to buyback shares). Note that this refers to the market as a whole; at any given time, some companies will be leveraging up or altering their capital structure towards more debt, but that's ok if only a few companies do it. It's a different matter when the market as a whole does it.
Buying back shares by using debt (i.e. more than free cash flow allows) is always a dicey proposition depending on how leveraged the company is. The last time companies used debt to buyback shares was in 2007 and things didn't turn out well. You can value-destructive actions like companies--many of the same ones using debt to buy back shares earlier--actually issuing shares at really low prices (because they fell so badly in the bear market of 2008) to pay back debt.
Having said all that, interest rates are extremely low--some countries have all-time lows beyond 5000-year lows (or basically since human recorded history). Debt should be easier to service.
Remains to be seen if this turns into a disaster...
(source: "Bondholders Left Holding Stockholders' Bag," by Lisa Abramowicz for Bloomberg. Jan 20, 2017)