More than likely some fund/institution selling out/lightning up on their position. Many of them sell at market driving down the price....these movements don't worry me. After they sold, buyers came in and moved the price up.
The shorts now are stuck between the rock and the hard place. It should kill most shorts before it rolls over. Our trading group has simulated millions of those crash-rebound cases over the history of all stocks. The program runs 7 hours continuously to complete such simulation. In 70% of the cases, when the first rebound leg started to weaken and the short swarmed it proved to be a bear trap. When the second rebound leg started to weaken the chance of roll over is much higher, but by then most shorts are already killed and many new longs are trapped.