Wait, Taylor Swift is Canadian?

So, why do Canadians have flappy heads on South Park …? Terrance & Philip (the archetypal Canadians in the show) were originally portrayed in a show within the show. They were poorly drawn as a parody of the actual South Park style, and Parker and Stone just carried that style forward when they had to portray them as “real” characters. That then propagated to every other Canadian. I suspect that it was a continuity issue that just took on a life of its own, rather than a conscious decision to style Canadians differently.