Technically it's not an IDB but this needs to be investigated. Could it have been that the ticket was cancelled as the passenger was at ticketing and got to the gate after the cutoff time?

An airline can't just cancel your ticket just because the flight is delayed. This would be unlawful and a violation of AC' s COC, which calls for rebooking ont their next available flight, under both the COC and Canadian law.

The airline is also required to provide a reason for the refusal of transportation. Is it possible that the flight was oversold and therefore the ticket was cancelled in order to avoid paying compensation?