Key para. in the article.
“It is important to note that it is not the rise in margin debt that is the problem for the markets - it is the fall. When the ultimate reversal begins, and investors are forced to liquidate to meet margin calls, the market begins to feed upon itself. This forced liquidation quickly accelerates downside reversions in equity markets leaving investors little opportunity to react. The last two peaks in margin debts have had nasty outcomes for this very reason.”