Bob-ten has pointed out a single buying house dumping shares virtually every day for months. My guess is the NOVA shareholders (it was privately held, probably only a handful of actual people who hold many millions of shares each) wanted out. When you want to sell that many shares you normally sell them at a discount to a trader at a brokerage and they proceed to sell them over time to try and scalp the difference.

We're also at a high-risk time for the markets in general. Loose monetary policy by central banks, particularly the Fed, ECB and BOJ have led to major asset bubbles everywhere. Most of the money invested in the markets are managed in balanced portfolios which rebalance regularly, cut exposure to sectors at risk, etc.