[Note: HistorySE might be a better venue for this question, if looking primarily for the motivations of American residents, either native-born or immigrant, that enlisted in foreign armies. G&FH SE answers focus more on documenting the individual, helping identify relevant records or providing resources for context].
Many of the volunteers in the CEF (Canadian Expeditionary Force, ~620,000 enlistments) were foreign-born - unsurprising in a "new" country, with a large proportion of immigrants. Patriotic feelings ran high, especially at the beginning of the war.
According to wikipedia's "Canadian_Expeditionary_Force" "By the end of the war in 1918, at least 'fifty per cent of the CEF consisted of British-born men'.
Googling american citizens in cef - yields a number of hits. According to at least one site, the blog "Roads to the Great War", 35,000 (~5% of CEF) Americans served in the CEF, many in "American Legion" contingents. U.S. citizenship was not at risk in WWI, and Americans were actively recruited in the earlier years of the war.
With 100th anniversary of WWI commemorations ongoing, many records are being made more available. Look not only for the official service papers through Library and Archives Canada, but look for histories of your grandfather's miltary units to flesh out what events he took part in.