1. Forced margin selling - those who bought at the top on margin are likely being forced into selling.
2. Longs who are taking a loss and don't see any near term chance of recovery and want to put their capital to work elsewhere.
3. Shorts - I know their risk/reward is not great at these levels, but this board was saying the same thing at $5.00. If you had shorted at $5.00 and covered today, you would have made a nice profit. If the markets continue to tank, who's to say that shorting at $4.29 and covering at ??? wasn't a good trade.