Due to the cold temps YYZ is operating at a reduced arrival rate, so I'm assuming AC has needed to trim it's schedule. Short haul flights are always the first to get impacted. AC and WS would always be impacted the greatest given they operate the most flights. AA doesn't have the same volume of flights, so they not be impacted...hence why they are still operating.

https://extranetapps.navcanada.ca/ois/ois.aspx