I have been thinking about this fact as well, and my answer gets to what I think actually caused the downturn.
The shorts were pushing it down, and it kept putting in lower lows, and it was so consistent, they never had to cover, this encouraged more shorting, and I think some huge size was put on with no fear as it was a safe one-sided market, so the shorts loaded up like we have never seen before, and then key techincal levels were hit, causing forced liquidations, triggering more market sell orders, and the only explanation for the quick buyback was the same shorts covering trying to lock in their gains once the damn broke.
This all comes down to probably the biggest short-term short size ever being put on a market, playing for the break, it broke , and subsequent covering........and the reason they didn`t stay short is obvious, who could have imagined such a break, you obviously take profits on that break, it is a once in a lifetime break, and the real pressure was in getting out with as much potential profit as possible when so many shorts were covering probably at market.