I sort of got the feeling, from the movie I mentioned above, that a large part of it was private brokers who were making these deals with home owners?. Those brokers would then just sell the mortages to banks after the fact. The banks would bundle them up and use them in their way overly convoluted way as a means to make money.

The banks made big money this way, and so they needed to keep creating more mortages that they could bundle up. So they incentivized brokers heavily to make these loans, and more so for variable rate mortages presumably so that they could make more money down the line. Eventually you work your way through all of the safe bets, so you start working down the food chain. So those brokers had every incentive to get people into houses any way possible, so that they could make big bucks selling the mortages to the banks.

Throw in a huge economic bubble which made everyone overly optimistic that they could make the payments, and the fact that they probably worked their way down the food chain gradually over time, and it's not so hard to see how it would all go down, without there needing to be any one all-power henchman making it all happen. It was just human nature and circumstance in a perfect storm.