How can the small number of people who bought houses under Comm Invest be to blame?

I often hear that the cause of the financial meltdown was people getting loans they couldn't pay back. Despite their small number weren't these loans government secured? Isn't the more likely culprit home owners of all cles who simply walked away from their properties when the bubble burst and their values started plummeting below what they paid for the property? ie foreclosure as a get out of debt card?