From the link:
Each time the levels of margin debt in 2000 and 2007 became unsustainable, the subsequent decline led to bear markets in which the S&P 500 index declined 40% to 50% (see the charts below), and now when it drops it will be dropping from an even higher height.
Can a 40-50% bear market happen again? You can bet half your portfolio on it.
Once margin debt begins to unravel, it will feed on itself — when the margin calls come, it is either put up more money or sell the stock. Selling the stock drives it lower and brings more margin calls. Nothing else will matter, not fundamentals, not news, not hopes, not dreams.
For more perspective, discussion and the charts:
THE DIVERGENCE THAT KILLS THE BULL