It's a few different factors.
For one, some major US midwest refineries have had issues over the past couple months. Since prices around North America are fairly well connected by established differentials, when a major producing region like Illinois has supply issues it ripples across the entire North American market.
Here's a pretty good Edmonton Journal article on the subject from late May.
http://www.edmontonjournal.com/Blame...656/story.html
The dollar also doesn't help at all. A 1.25 exchange rate eats up a big chunk of the differential between WTI in USD and gas prices in CAD.