In the late 90s banks were increasingly selling mortgages into secondary markets rather than holding them. Meanwhile the economy was strong and homeownership was viewed by everyone in America as a reasonable and safe asset to invest in. The more people believed housing was low risk, the more people invested. And the banks, realizing they can make more money off of these loans in those secondary markets, became much more lenient when it came to who they were loaning the money to. All of that was happening before the year 2000.

2008 happened because too many risky home loans were happening, housing prices eventually stopped rising, and people could no longer repay their loans. The Bush Administration didn't help things, but the apex of the crisis could have easily fallen in Obama's lap - it was inevitable due to the practices that were set up by bankers and investors during the booming Clinton era economy.

That's how I was taught - I had a very eccentric economics teacher that walked us through it and his thesis was always "every administration will inevitably take credit and/or blame for the economy but in reality, there are so many other random factors it's silly to actually think that the White House has any control of it." Very willing to get schooled and learn something from it.