Because of the mad cow case. However, there are sugestion that mad cow is not only a Canadian problem, because the infected bovine could have came from the US during 1998 import of Black Angus (over 70% of Black Angus came from the US to Canada).
It is hard to speculate what the true motive of the US in this situation, but the main thing that we know Canadian beef industry will never fully recover from this, just like the Canadian logging industry.