Some say we are about to drive over a debt cliff - by what empirical data and measure do they base this?

Chicken little prophesy on? What magic percentage of GDP is the breaking point? Do they base it on a quarter century of 200 PLUS Debt to GDP ratio Japan has PROSPERED with? Do they base it on the collapse of EU states that are shackled to the Euro? If "Deficits don't matter" a few years ago.. why is it all of a sudden a national emergency?