Briefly a bunch of tech savvy traders discovered that the trades they were trying to make for their large investment clients were being messed with by the high speed traders. The purchase orders would be intercepted by the high speed folks then their computers would place orders for the same stocks ahead of the original order. They managed to skim off billions using this legal scheme. The new guys on the block have started their own exchange which is doing well. The people who continue to trade as before are still getting ripped off.
http://www.cbsnews.com/news/stock-market-rigged-says-michael-lewis-in-new-book/
http://www.cbsnews.com/news/stock-market-rigged-says-michael-lewis-in-new-book/