The housing bubble was the fault of people taking out mortgages (and bank's lending on mortgage loans) that they couldn't pay, and did not put enough equity into the property to begin with. The bank's were under pressure to lend to (and are beginning to be pressured again by the current administration) lower income borrowers and to stretch the traditional underwriting parameters to meet the needs of the community, inclusive of lower income borrowers. The biggest change being the limited amount of equity required to be paid down to purchase a home or other property.
Lower income borrowers tend to be disproportionately minorities. So, they didn't default because they are minorities, but more of them were minorities than in the general population.
But there were also certainly plenty of wealthy people that became over-extended due to teaser loans and relaxed lending standards on higher-end properties too.