Long time listener, first time poster.
Read a very interesting essay on what actually takes place on trading floors these days.
I remember hearing a long time ago Rym talking about how one of the markets dropping spontaneously and then correcting itself.
This could be the reason, bots! Or more precisely algorithms that place buy/sell orders. There are competing "sniffer" algorithms that can track buy/sell algorithms and buy the same stocks slightly faster and sell it back at a profit. Lastly there are "spoofing" algorithms that fool sniffers by placing fake buy orders of stock that it wants to sell, driving up the price and selling at profit.
The spoofing algorithm is illegal, but sniffers aren't. Really makes you wonder if capital markets really work when you have bots trading shares, independent to the outside world and are just trading to make money.http://www.lrb.co.uk/v33/n10/donald-mackenzie/how-to-make-money-in-microseconds