I'm actually surprised that the decline comes out of car trips, not out of plane trips.

 Truth is, Canadian dollar stayed relatively stable - or gained outright - against some other foreign currencies, and the current airline ticket prices to US are not that different from flying to Europe or South America or Asia. A car trip to US can be justified simply by the fact that there are no foreign countries that can be reached by car, but in airline world US has long been a questionable choice.