This is actually very interesting. It may have been led by China. Chinese market fell on Monday due to reserve requirements change (though the slow decline actually started quite a bit earlier), and it was widely reported on Monday night. Commodities followed on Chinese news and European debt on Tuesday; Europe continued falling on Wednesday on Greece and Portugal speculation, then this went around the world one more time and was reported at about 3am today (emerging markets decline), and then later in the day as decline continued. Speculation on European state debt compounded the issue (witness the Spain rumours). Then at 13:10pm El Erian was reported to have said that there is a possibility that European banks will stop lending, at which point the US markets hiccuped and went into accelerating decline (look at Russel, SPX and Dow from noon onward).

I will try to add links when I get home later. Clearly it was much more complicated than this, but this pattern of falling dominoes going around the world is at least suggestive.

Update:

China reserve requirements:

http://www.businessweek.com/news/2010-05-04/china-s-stocks-d...

Commodities, etc.:

http://www.google.com/hostednews/ap/article/ALeqM5jpti0ArQEl...

http://www.theaustralian.com.au/business/markets/global-mark...

http://www.businessweek.com/news/2010-05-06/corn-soybeans-wh...

Emerging markets:

http://www.livemint.com/2010/05/04214121/The-Chinese-contagi...

http://online.wsj.com/article/BT-CO-20100505-713490.html?mod...

El Erian - the best I can find is this. The lending comment was attributed to him when I saw it, but here it is a "trader speaking on condition of anonimity".

http://www.cnbc.com/id/36992469