The facts don't really support your conclusion. If you look at the blockchain, there were an influx of Nxt into the exchange before the price drop, but from dozens of accounts. A few bigger amounts (two 2M, a few 1M and less), and many various smaller transactions. It "obviously" came from different people. Unfortunately Dgex deposits are processed manually, and there were an almost 12 hours gap. These deposits hit the exchange the same time, most people were waiting hours for them to arrive, so in was a frenzy of sell orders (and bought up buy orders) that pushed down the price.