Mooseman - ultimately, that amounts to the same thing and it doesn't matter which "class" caused the problem. How did they get low income people into housing that they couldn't afford? By introducing new types of mortgages and loans. Historically, the original purpose of these loans was to get more people into homes. However, by introducing those loans with the intent of allowing low-income people to buy houses that they cannot afford, at the same time, it opened the door for people who could afford a house to purchase a larger house than they realistically should have.
This didn't lead to an immediate boom anyway. That takes time, but it did start artificially inflating prices. As that continued, people started realizing that housing prices were continuing to increase artificially and the speculators got involved. People would buy a house with a serious balloon payment that they knew they wouldn't be able to afford, but they didn't care because once the loan came due, they would have enough equity in their home to refinance with a more reasonable loan.
The more this happened, the more the market faciliated it and the faster housing prices rose. But that ultimately wasn't sustainable because it got to the point where there weren't enough buyers to keep things going.
The crash happened much faster than the boom. At first, housing prices continued to rise, but they couldn't rise like they had before because there weren't enough buyers. Some of those loans came due and speculators and home-owners alike weren't able to refinance like they had hoped, so they ended up losing their homes. Once that happened, supply is now going up without a significant increase in demand, so housing prices start dropping. As they drop, the problem is compounded because more and more people are unable to refinance. In many cases, people who were careful about the type of loan they had lost their job or whatever and were in the same boat because they were upside down on their home through no fault of their own, but became part of the spiral.