Most likely it was all algorithms. You'll note it was a slow downtrend then it hit a window that triggered massive selloff cascade. When it hit the bottom from February the other algorithms were set to buy. I am betting few humans got through an individual order during this. It was mostly software.
This is what cause the crash in the eighties, we have some fail-safes in place to slow it but most trading is done by tuned algorithms during stuff like this. In fact, the PPT (Plunge Protection Team) might have kicked in...
I wonder whether a large enough entity could actually deliberately hold and then suddenly sell enough of an instrument they believed to be undervalued, just to ensure their competitors stop losses executed, then immediately buy back the entire market for the instrument.
Not really. Porsche bought lots of VW stock either outright or as a call option. They also lent their VW stock to short-sellers. This way they `recycled' some of their VW stock, because those short-sellers sold to Porsche again.
This allowed Porsche to build up a huge position, with only a few willing market participants as counter-parties. They also managed to short-squeeze the short-sellers. (http://en.wikipedia.org/wiki/Short_squeeze)
Riots because of Eurozone-pushed austerity measures; Greek citizens would (understandably) rather not cut services than comply with a largely foreign monetary policy.
There have been riots in Greece for at least several days though - photos of them have been showing up in my Facebook feed from a friend who is in Athens right now. I'm still not sure that explains today's sudden loss.
