In this instance, I think shares were simply withdrawn from the pool rather than shorted. According to my volume charts, just 400-500k shares were traded during this period, meaning that 1MM could not have been shorted. I have no idea why they would do this, other than to perhaps try to gauge demand or drive the SP down through intense shorting ahead of (good) earnings. When the shorts don't take (all) the morning bait maybe they pull those shares back.
Remember, borrowed shares != shorted shares.
That is why I specifically wrote "borrowed", not shorted. The similar thing is happening in pre-market, a lot of shares borrowed, so short sellers have enough of a supply to sell when they see fit...