It was unable to raise money on international bond markets due to the massively high interest rates demanded for its bonds a reflection of the highrisk of default that investors see in Greece.
If you put your whole bond portfolio into the Vanguard index fund, which is mostly in U.S. governments, you have a fairly low risk of default but a fairly highrisk of not achieving a positive real return.
Layering of risk means the borrowers courted by these lenders had multiple characteristics that would put them at highrisk for default, including no down payment, no documentation of income or assets, low credit scores and second mortgage in lieu of a down payment.