It's not terribly surprising that the London attacks struck a nerve in Canada. The ties are historical and deep.
I think the deep root of resentment from Canadians is the fact that the US raised up arms and rebelled against the British in 1776. Canada is in fact what the US would become if you decided to stay within the British Empire.
According to their minds you have betrayed the cause of the British Empire. Even superficially the Empire is no more it still survives on a subconscious level among English Canadian elites. I imagine it is most acute in none other than Ontario: Ontario is the English Canadian province with the most elites, the most anti-US trend, and the most widespread pro-monarchy sentiment. Support of the monarchy is the highest in Ontario, believe it or not.
The root of Canadian anti-Americanism goes two ways: yes, some are leftist sentiments, but a deeper cause is because Canada still supports the concepts of the British Empire and Commonwealth and "It is the breakaway United States that caused the destruction of the Empire. We will support whatever causes, left and right, that brings the US down!"
In other words, it is a bit like a previously good student beaten by an even better one, and he wishes the best students to be brought down. A common and understandable, but hardly noble, sentiment.