Does this necessarily have to be a "real" seller?
During the three days preceding last Friday, about 1 million shares were traded.
Each day, ARCA, CINN, INCA, and/or their customers sat on the Ask, and took us down to the 13.50's. In many cases, they dropped 100 share bombs at the Ask, but the same MM's/ECN's were waiting at the Bid with offers for 2,000 shares and more.
They've been doing to the same things for days (including today), and it would appear they could be selling the shares they bought at 13.50 last week.