Post civil war, there was concern that Canada would be manifest destinied. Britain was somewhat supportive of the CSA and Alaska was purchased in 1867 so there were reasons to think they were next.
Well, TIL. Interesting that I had so many hours of Canadian history yet this particular angle didn’t stick with me. I recall a framing of the constitution act as a quintessentially Canadian “we asked politely for independence, and Britain said okay”, but after some reading it does seem your “let’s get out of here, the neighbors are psycho” framing also holds water ;) I suppose both are true.