Rather than identifying exactly "how", I think it's more of a general problem of greed and lack of oversight.
1. Excessive leverage = low/none reserves to handle defaults.
2. Low interest rates encourages excessive borrowing/leverage.
3. Elimination of "qualifications" to get a mortgage loan.
4. Regulators(?) not saying, "Hey, that's very risky and is not allowed". You know, like liar loans and layers of derivatives where nobody posted reserves for possible obligations.
If the system lets some run amok plus some assume they're too big to fail and will be necessarily bailed out.... well, many will find a way to be stupidly excessive.