The mechanism is renewal risk. A borrower who locked a rate several years ago and is renewing now can see a materially higher payment, and CMHC's underwriting still tests every insured application against a gross debt service ratio of roughly 39 percent of income and a total debt service ratio of 44 percent for exactly that reason.
Those ratios are designed to leave room for a rate increase at renewal, which is why they exist even when a mortgage was affordable at the original rate. A household that qualified with less room to spare is the one most exposed if income drops or a term ends at a higher rate.
Whether delinquencies are rising in any given month matters less to a seller than whether their own renewal math still works, and that is worth checking early rather than at possession. Get your hand-reviewed valuation from Joel Dyck.