Throughout the bulk of 2013, investors became accustomed to seeing U.S. stock market benchmarks close near their intra-day highs.

In August, however, we may be witnessing the birth of a disconcerting pattern whereby institutional investors sell broad-based equities into strength.

Consider the trading activity on seven of the most popular ETFs on Tuesday, 8/20.

Tuesday’s gains did snap a 4-day losing streak for the S&P 500.

Yet the Dow Industrials logged a 5th consecutive loss, and that came after trading in positive territory for the majority of the session. Moreover, money flow data at show that adviser-based investors dumped roughly $150 million of iShares S&P 500 ( IVV) on heavier than normal 3-month volume; similarly, Vanguard loyalists bid farewell to $78 million of Vanguard Extended Market (VXF) on 7x the typical volume over the last 3 months.

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