It's pretty complicated... I'm not sure I can explain it.
When interest rates are low, this increases the incentives for people looking to buying a home. Real estate (and other financial assets, such as stocks) prices increase in value as interest rates decline (imagine there are more buyers of houses, and more people willing to borrow to invest).
Subprime mortgages are generally higher risk borrowers. These are people with bad credit. They have to pay higher interest rates. While real estate prices are going up, and the economy is ok, it is very profitable to lend to this group (they have high interest rates and are able to pay). When the economy gets weaker or interest rates, they are at a higher risk of defaulting on their debt.
There was a significant amount of this type of lending in the US in the past several years due to low interest rates, and the real estate boom.
The crisis comes partly from the extent of this lending and the financial derivatives based on these loans. Many banks were exposed to this risk. Now that the higher mortgages are starting to default, many banks have been forced to write down very heavy losses. In addition, the liquidity in many of these derivatives was very weak, so asset managers sometimes found themselves with an instrument that was declining in value and also illiquid.
What this has to do with learning English, and why I decided to write this, I have no idea.