It's largely a matter of supply and demand, with the recognition that "dollars" (and other forms of currency) are just another product.
Since about 1995, it's been easy to get dollars. Interest rates have been low, so present dollars have been easy to get for the promise of future dollars. With an oversupply of dollars, their value drops in relation to other products, particularly whatever the investment vehicle of chioce is (tech stocks, real estate, minerals, etc.)
Combine this with a herd mentality -- people wanting to invest in whatever's "hot" -- and dollars keep flowing into one particular sector long after it's no longer a fundamentally good investment. The bubble inflates, and then eventually bursts. For a short period of time, dollars are hard to come by (and those who have dollars can often pick up assets on the cheap) as everyone who just got burned hoards whatever they have left... until they go chasing the next hot investment.
The housing bubble was particularly nasty because, not only were interest rates low, but banks were loose with their lending standards because they weren't directly on the hook for the loans they gave out. This allowed people to get a lot more dollars than they should've been able to pick up -- big housing loans with no down payment and no proof of income. Unsurprisingly, this led to housing prices rising well beyond sustainable levels, and then to a lot of people not being able to pay off their loans. All because people had easy access to money.
Since about 1995, it's been easy to get dollars. Interest rates have been low, so present dollars have been easy to get for the promise of future dollars. With an oversupply of dollars, their value drops in relation to other products, particularly whatever the investment vehicle of chioce is (tech stocks, real estate, minerals, etc.)
Combine this with a herd mentality -- people wanting to invest in whatever's "hot" -- and dollars keep flowing into one particular sector long after it's no longer a fundamentally good investment. The bubble inflates, and then eventually bursts. For a short period of time, dollars are hard to come by (and those who have dollars can often pick up assets on the cheap) as everyone who just got burned hoards whatever they have left... until they go chasing the next hot investment.
The housing bubble was particularly nasty because, not only were interest rates low, but banks were loose with their lending standards because they weren't directly on the hook for the loans they gave out. This allowed people to get a lot more dollars than they should've been able to pick up -- big housing loans with no down payment and no proof of income. Unsurprisingly, this led to housing prices rising well beyond sustainable levels, and then to a lot of people not being able to pay off their loans. All because people had easy access to money.