IMO, it's all a combo of accumulation, people who sold yesterday for a to buy back in lower today, people topping up, algo's pushing the price lower to get people to panic, and offloading yesterday's gains like Gharp mentioned. Algos / bots / hft usually is doing most of the work between 11:30am and 2:30pm I reckon, buying below vwap and selling above it. Usually no straight lines up or down. All my observations, I'm no expert.
