Spending more than you can pay back?
Buying more than you can afford if you are laid off?
Lending out more money than you take in? (banks)
Being behind the eight ball in a bubble situation. (housing)
The government trying to make home ownership available to everyone who shouldn't have been approved? (greed and falsifying applications by lending institutions)
ALL THE ABOVE! Plus many others including health problems.
Other than unexpected costly health problems, the bottom line is FISCAL responsibility.