A lot of it was people doing speculation in the housing market. The rules put in place by Democrats on mortgages gave rise to this. You had mortgages that were:
Zero down
Interest only
ARM's
Second and thirds
What all those did was allow people who had homes to buy a second or even third home using those sorts of mortgages and rent them out. That paid for the mortgage on the rental. I knew people back then up to their eyeballs in mortgage debt that owned 2, 3, even 5 homes as rentals financed on multiple mortgages off their first.
For people on a zero down, interest only mortgage, when the market collapsed, they simply walked away and let their house go. They had zero invested in it and wrote it off.
All of that was supposed to help the struggling and low income, but it only fueled a speculative market built on nothing but credit--think the stock market in the 20's that led to the Great Depression--led by middle-class and upper middle-class investors who were small fish wanting to become big fish in the pond.