Most people are financially responsible. But even a small percentage can create a financial meltdown.
A bank may have 5-6% in capital, perhaps lower. For a $1 billion bank, that is $50-60 million in capital. Let's say half that bank's assets are in consumer loans and home mortgages, and that default rates are 20%. That means $100 million of the bank's loans are in default. If the underlying collateral is less than half of the loan amount (which in today's market is reasonable to assume), then the bank may be insolvent even though 80% of their customers are making their loan payments.
If the bank has all its assets out in consumer loans, as many do, then a 10% default rate can ruin it.
I think total mortgage debt in this country is around $10 trillion, but I don't have a source for that number and it may include all mortgages, not just single family homes.