In Canada, they don't have fixed 30-year mortgages like in the US. They have 25-year mortgages with 5 year terms. They need to refinance within 5 years. If they take a 20-30% hit in the house costs, they will absolutely be unable to renew their mortgage, unless they come up with the cash to make up the difference in appraised value.
This will lead to a lot of forced selling or foreclosures which will only pull the house prices down further, and cause more forced selling.