The 10 year treasury yield has recently strongly overtaken the S&P 500 dividend yield. That aggressive shift is guaranteed to end such a bubbly bull market run.
https://assets.bwbx.io/images/users/iqjWHBFdfxIU/i9Zchv.nY_N...
The treasury move since September is one of the most aggressive of the last five years.
What happened is simply an inflection point. Markets operate heavily by trigger points. Those are getting taken out, sparking reactions (selling, asset allocation adjustments). The Dow went up three thousand points in two months, after the market was already at bubble price levels on multiples.
Shiller PE:
http://www.multpl.com/shiller-pe/
These valuation levels are batshit crazy. It doesn't take much to crash something that over-extended.
Let's be realistic here. Where was this market going from there? Dow 36,000 by 2019? A 45 PE for the S&P 500? It'll already take ten years of 3% US GDP expansion and higher global growth to bring the S&P's earnings multiple back to being close to reasonable.