In considering what could trigger such an event, it is important to keep in mind that the current level of stock prices is apparently being largely driven by investor sentiment, rather than by fundamentals.
For example, those high-momentum stocks, which draw an enormous amount of trading volume, have absurd price/earnings ratios and suffer/enjoy wild swings based solely on "mo-mo." In the event of a meaningful decline, those stocks would likely lead the way down since their investors would have the most to lose. Beyond that, there are quite a number of S&P 500 stocks with trailing 12-month price/earnings ratios in excess of 20. What will happen to the S&P 500 if/when those shareholders start running for the exits?