A large part of why the CAD has been weakening (against the USD) is because the value is crude oil has been falling, and Canada is a big exporter of oil.
But this oil doesn't belong to Canada. It belongs to the private companies that extracted the oil from the ground, such as Suncor Energy, EnCana Corp, and Canadian Oil Sands.
So if the value of crude oil falls, then these oil companies become less valuable because their inventory of oil decreases in value.
If the oil company's oil-inventory decreases in value, then the company's equity decreases (as "assets = equity - liabilities").
If the oil company exports this less-valuable oil, they'll receive less money for it, as they'll have to sell it for a low-than-usual price.
So if the oil company's equity decreases, then the company's shares decrease in value, leading to a decrease in the company's market capitalization.
Where does the drop in the value of the Canadian dollar come into play?