There's another possibility that's not as bullish as someone in the know simply stepping up and taking what they can....

It could be a margin call on a short seller with a significant number of shares that needed to be covered.  As the PPS climbs shorts have to fund their margin accounts, but if they're either unwilling or unable then the brokerage handling the short can simply take the funds in the margin account (there's a large excess to gaurd against a big jump) and cover the position off.