After a 20-25% drop from pre-earnings levels, those who bought on margin at higher prices are going to be getting margin calls from their brokerages. The either will have to wire in money to cover the margin call or be forcibly liquidated by the brokerage.
That explains the selling on an up day (today) and more to come tomorrow/Monday. Margin calls are due 3-4 days after they were triggered. The big drop after earnings from $16+ to $13 occurred on Tuesday, so that's when margin calls were triggered.