Oh, there's no question that the creation of slice-and-dice MBS's and a lax regulatory environment set the table- but it's also the case that Fannie and Freddie's collapse was foreseen at Treasury years ago and nobody took action. The result was fractional-reserve dominoes that fell in a tsunami of, in effect, cascading margin calls.
That's the tragedy here: there were plenty of warning signs, but few heeded them. As long as the gravy train (and the campaign cash) was rolling, the coming meltdown was ignored.
Geographically? What we find in those areas is that (1) high sunbelt growth rates combined with (2) very, very high rates of minority (Hispanic) subprime lending. In Southern California, in particular, La Raza was deeply involved in pressuring and/or suing lenders to increase their subprime portfolios in the name of 'fair housing.'
In the meantime, Countrywide and Golden West and all the rest came to believe that they could lend to unqualified borrowers risk-free, since in actual practice Fannie and Freddie would buy anything, completely ignoring their stated 8 in 10 rule: all the incentives at F&F in the early 2000s were to 'pump up the volume,' and the Congressional hearings revealed how F&F staff were encouraged through bonuses and 'production quotas' to rubberstamp every piece of junk paper that crossed their desks. So why wouldn't Angelo and the Sadlers push them? Let Uncle Sam hold the hot potato!