I think the largest component comes from this: "Now, most financial firms also invest for themselves. They use partners' or shareholders' money to place bets on stocks, bonds and other securities -- so-called "principal transactions." Merrill and other retail brokers, which once served individual clients, have ventured into investment banking. So have some commercial banks that were barred from doing so until the repeal in 1999 of the Glass-Steagall Act of 1933."

As a very small example, brokerage houses are of course prohibited from using knowledge of a client's impending activity to jump in ahead of a clearly stated intention. But if they have a huge client that has a pattern - e.g. buying or selling in chunks, at intervals, then it would be hard to prove they were making their own decisions because of that confidential knowledge.

I also read recently that one of the huge investment banks was recently putting together a massive acquisition of some kind, gathering mega-investors to make it happen, while failing to disclose that they were shorting the same investment. I'd have to google a long while to find it again.