I don't know why, so I can only guess. But I personally don't think it was intentional, it might've been just a side effect because of the criteria they use to trigger DS-5535. Just for example:
- Maybe it happens that Canada accepts more people with backgrounds on the Technology Alert List (a bunch of medicine-related backgrounds, robotics, AI and vaguely related things like software development, etc), and those are the people who then also end up applying for US visas.
- Maybe people in Canada happen to be from a more diverse set of countries they were born in, and some of those countries might be scrutinized more. Somewhat related, maybe the names of immigrants from Canada are more diverse and happen to be redflagged more as a potential match to known criminals.
- Maybe there happens to be more information collected by the government and various agencies about people from Canada, and the more data there is about you, the more probability some minor thing ends up triggering the automated "red flag".
And it might be not just one single reason, it might be a combination.
Anyway, it's all just wild theories, and your guesses are probably as good as mine.