In his 1903 speech "I Believe in a British Empire" advocating for tariff reform, Joseph Chamberlain made the claim that the United States was seeing a mass emigration to Canada. I can't find anything on this and have two questions:
He's likely referring to Americans who emigrated to Canada's western frontier in the early 20th century.
Yes this emigration of US citizens to Canada was real, particularly in the provinces of Alberta and Saskatchewan. About 20% of Canada's immigrants at that time were from the US. That number is much higher on the prairies. American farmers and ranchers mostly came for the lure of cheap land.
They could buy more land, for cheap, that's why most came.