Its really simple. Some large holder needs to liquidate for whatever reason.
The specialist/market makers have become aware of this, and are using thin trading to extract as much from this seller as they can.
You will know the seller has fully liquidated when the price just starts coming back up for no reason.
When I was younger and inexperienced, I would place large trades on stocks. I noticed that the simple event of seeing a large purchase from someone with an etrade account was enough for the specialists and market makers to drop the stock price afterward, and yes, I admit they did spook me into selling back at a loss. They simply assumed a large purchase from an etrade account meant a margin buy or an easily spooked investor and basically tried to margin me out of the position or scare me off with a sudden unaccountable and scary price decline.
In the case of these long slow sell offs, its some big hedge fund or institutional seller or large trader who really needs out for whatever reason. Rumours around the street or just watching the price action is enough for the specialists and market makers to realize that they have a seller over a barrel.
Funny how some other large players see's what's going on and has opened a huge call position in anticipation of that this big seller will finally be out soon enough.