A little geeking found this
at about 3min.
I was also thinking that it might be for the higher weight, I guess the with the increase in engine thrust Canada were hoping to carry more, especially as they would be using it as their primary fast jet. My PoF is quite basic but the more I think about it wouldn't any increase in weight affect all the undercaridge to the same extent, therefore no change in AoA.
Could it have been to do with the wing shape for take off with slats and some flap deployed giving a negative AoA without the boost?