Uh, what? The government did not "force" banks and mortgage firms to provide high-risk individuals with loans. The banks and mortgage firms did it all on their own - the risk was deemed worth it, as they would simply increase the interest rates over time (or start them high from the beginning), and thus make a lot of money as long as the owners were able to pay. They didn't require down payments, they decreased (or often waived) the general requirement that your monthly loan payment be no more than 30-40% of your monthly income. They would then turn around and bundle the mortgages up into securities (the "high-risk securities" that people talked so much about in 2008), and sell those off as a way to attempt to rid themselves of the potentially bad debt.
Of course, other large financial institutions were the ones purchasing those securities, and so it was basically a giant game of Russian Roulette, in terms of hoping the bullet never came up.
Ultimately, once those adjustable rate mortgages (which the banks themselves were more than happy to provide - the federal government didn't "force" anything upon them) started increasing, people found they couldn't pay the greatly increased monthly payments (usually for a home). However, as more and more homes were being built, the boom in housing prices collapsed, so that homes could be purchased far more cheaply. This resulted in those with adjustable rate mortgages being unable to re-finance their loans, and thus faced with the inability to make their increasingly-larger monthly payments, many defaulted. With those defaults, anyone holding those mortgages and loans suddenly saw the influx of payments dry up, and thus the market collapsed.
One thing drove the boom (and then bust): greed. It was a carefree time, beginning around 1999/2000, and every bank and firm were trying to bring in as much money as possible. That is capitalism's way, after all. Unfortunately, their figurative moneytree had its root on loose alluvium, and once the earthquake of defaults began, that tree quickly toppled over. Don't try to place the entire blame (or even the majority blame) on the government and regulation. If anything, it was deregulation.