As someone who "accidentally" got my first rental property in 2007, I saw this first hand. My wife and I had lived in, and rehabbed, an old house that we bought for very cheap in a rural area several years before. In 2007, we moved away to a new area in order to get better paying jobs.
We listed our old house for sale and purchased a new house in the new town. With the way too relaxed underwriting requirements, we easily qualified for our house. I was shocked at how much we were approved for. Something like $350,000 while carrying our small mortgage on our other property. We instead went with a $200,000 house, with no money down. Looking back, there was no way the bank should have even considered that loan, but like others said, everyone got loans then, even if the math did not add up.
While our other house was up for sale, everything began to crash. Our old house did not sell, and our new house was way underwater. Fortunately, we used traditional 30 year fixed loans on our houses. If we had financed with an ARM, we would have been in deep, deep trouble.
Eventually, we began renting out our old house. That made all the difference. We immediately had cash flow. Even through we could not sell it in the market, we made a modest cash flow.
Now, our new house was supposed to be something of a transition house. We were actually planning on selling it once we got settled in our careers and started a family. But everything crashed, including our home value. We spent the next 10 years (basically until now) trying to get equity in the place. At the same time, we worked hard and increased our income at our "normal" jobs.
At this point, we are finally turning this house we currently live in, into a rental property (Thank you bigger pockets!). We just got a loan on a new place that is bigger than our current house and was ironically, cheaper than what we paid for our current place in 2007.
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As a side note, the situation that created the "ready to burn" fuel for the crash was pretty much what was outlined in the big short. We also had government basically telling banks to give away loans. There was this "ownership society" thing that was pitched in politics as something that would reform and save America.
All this fuel needed was a spark. People were way over-extended. When oil prices soared (gas went from sub-one-dollar to four dollars+) it was all that was needed to cause people to begin to miss payments. As soon as that started happening, it began to snowball. I know of at least a couple people, personally, that you could trace the rise in gas prices as the catalyst that pushed them over the edge. Even companies began having negative profit due to the spike in fuel prices.
Soon, consumer demand dried up, as everyone was underwater on mortgages, credit cards, etc. You ended up with a situation that just needed to deflate. So much debt was issued on false expectations. And all this time, basic necessities went up in price. During the height of the recession, we had used car prices spike (due to cash for clunkers), corn prices spiked (due to ethanol mandates), and oil prices spiked.
Everyone was just so over-extended, there was no margin for decline in "profit" in their personal finances. Everyone kept saying things were going up and up, so no need to plan for the downside.