I'll admit it's my personal estimate, looking at last decade's constant up-up-up of US stocks, especially the tech sector, and (as mentioned in the article) the widening gap between the value of U.S. household assets and GDP growth.
It's unsustainable growth, and I'm afraid the crash is going to happen sooner rather than later.
Will it be as bad as in 2008? I don't know. But it's going to hurt (except if you're wealthy enough to invest heavily right after and ride the wave back up again).