Basically a lot of their costs are is USD, but most of the revenues (and the accounting) in CAD. You'll always end up with forex gains or losses, just no one knows ahead of time which way the exchange rates are going to go. And they are way more volatile than they used to be.
I read this somewhere else: "Much of AC's debt is in US dollars and at the end of every quarter it has to mark the value of the debt in Canadian dollars. So fuctuations in exchange rates will cause earnings to move up or down. Because this is a non-cash artificial charge, you should look at earnings before foreign exchange variations."
I am not an expert by any means but I think it would have to play into this. Concern by the traveling public choosing other carriers ... unsure how big an effect, but I am sure it must have some.