The financial crisis of 2007-08 was largely a result of legislative changes to the Community Reinvestment Act, namely the Gramm–Leach–Bliley Act. These policy changes essentially made it easy for us to heavily leverage our some of our assets, namely housing. The result was a bubble in home values. What goes up must come down (usually), and the rest is history.
After the 2008 market crash many folks pointed to various stock market cycle theories, demographic theories, various patterns, etc. They began making predictions. Online postcasting prophets appeared out of nowhere, foretelling of certain woes in the years to come.
At this point, it's safe to say that most of them were wrong, since tomorrow is the last market day in 2017 and most predictions I read fell into the range of 2014-2017.
We can all stop worrying, because none of these guys knows what they're talking about. When it comes to the future, only one thing is certain: the market crashes on 08/09/2019.