Its pretty simple however. When a stock goes down, there are more sellers than people wanting to buy. When stock price rises, there are more buyers than sellers. In either case the price adjusts to an equilibrium, the point where buying equals selling.
At the time of my post, the stock was down 8% on the day. As it turned out, it only finished down 2%. There was increased demand in the last couple of hours. Volume was about 500% of the recent averages.