What Happened At Yesterday's Close

From CNBC:

Here's what happened: fear levels are so high that traders are reluctant to even consider buying into the close. But many bulls can smell that the market is oversold, so they want to buy, but they need some signal, even a feeble one.

At 3:40 pm ET each day Market On Close (MOC) orders are posted that indicate if there are unusually high levels of buy or sell orders for individual stocks at the close. The purpose is to let market participants know what kind of orders are around and to attract buyers or sellers to imbalances. These orders are revised again at 3:50 pm

These MOC orders normally make little difference in the market close, but with fear so high investors look for some sign of what is going on. When they saw that the MOC orders were relatively benign (about even buys to sells, not heavy volume), they came in and bought heavily.