I believe you are right about an Institutional Investor selling their exposure. While I did not see the volume all day, when I checked at 11am 100,000 shares had been traded. Then, when I checked at 3pm the volume had spiked to 1.3 million shares. Would not surprise me if there were several multi hundreds of thousands of shares trades today. Even under such selling pressure the stock was only down less then 1% for the day.