Since the repealing of the Glass-Steagall Act in the late 90's most of the remaining investment banks or financial institutions are involved in both lending and trading.
The investment game being described in the article "The End" encouraged the market to work in unscrupulous ways in order to get as many loans as possible. The "lenders" were engaged in activity that pushed homeowners to buy, refinance, HELOC, or take part in any other transaction that could later be sold on the securities market. Since the rating system was being manipulated, the securities could then be sold to unsuspecting investors.
Widespread fraudulent behavior, from appraisers all the way up the ladder to "Wall Street" moved the market values to unprecedented levels and now they've come crashing down.
This Countrywide Consent Judgment shows some more of the shenanigans.