The original source: https://www.oag.com/blog/canada-us-airline-capacity-aviation-market

Using forward booking data from a major GDS supplier, we’ve compared the total bookings held at this point last year with those recorded this week for the upcoming summer season. The decline is striking — bookings are down by over 70% in every month through to the end of September. This sharp drop suggests that travellers are holding off on making reservations, likely due to ongoing uncertainty surrounding the broader trade dispute.

It’s also important to note that this is more than just leisure travel between Canada and the US itself.

I don’t necessarily think these are the main driving factors, but you could attribute some part of this to:

1. economic recession, because firms oftentimes cut back on travel in their budgets as the first line items to be cut (prior to layoffs), and businesses may be more reluctant to hold conferences and large meetings in-person during periods of economic stress, and fewer business negotiations/meetings are happening due to tariff anxieties.
2. declining air traffic to the US overall because of visa worries, the proposed travel bans or spite - Air Canada + airline alliances competes with US airlines for passenger bookings (i.e. itineraries like London -> Toronto -> Kansas compete with equivalent US itineraries of London -> New York -> Kansas ), and visa policies like the China Transit Program exists to help Air Canada and the Star Alliance leverage Canadian airports as transit hubs to the US. Remember: if privileged Canadians are scared about being allowed entry to the US without being detained in an ICE holding facility, you imagine how citizens of developing countries must feel about traveling to the US right now.

The trend only holds true until September according to the source, so general uncertainty definitely seems to be a key driver here.