It's all about cycles and patterns, which tend to repeat over and over throughout history.
As mentioned prior, this is one of my favorite tables, done by Yardeni Research. It shows the bull and bear market cycles for the S&P 500 since 1929.
Even during the 17 year massive secular bull market run from 1982 to 1999, there were several pullbacks over 20%.
Since the 2008 financial crisis, there has only been ONE significant pullback nearing a "bear market" correction of 20% (according to the table, it was in 2011 at 19.4%).
In other words, since 2011 there has been no bear market pullback of around 20%, and therefore the probability increases of having one in the near future, perhaps post-elections or in 2017, which would confirm the cycle and pattern of the past 80+ years.
http://www.yardeni.com/pub/sp500corrbear.pdf