Why do you think some big entity is doing the selling? Maybe it's a ton of smaller transactions?
Depends what you consider a smaller transaction. I've seen orders of 500+ BTC appearing in seconds one after another. Those who invested at 300+ got scared that it's not going up and dumped it all back to the ~280 which caused a panic sell that broke below 260...
Check bitstamp for example the price was slowly sliding with some resistance to $280 and then dumped to 255 almost instantly (panic sell). panics usually have a rebound when they end, so the price should go back to ~290 in the coming days.