Well...Yang wasn't patriotic (as far as I know); after all, Heretic Huang, a bird of the same feather as him, used to regularly post rants against the government. And Guo had made it perfectly clear that he found the Song emperor and Song ministers to be perfectly contemptible; what he was defending was the people, not the emperor. So, it's not too much of a stretch, if we consider that Zhu Yuanzhuang was doing horrible things to Wulin/the people.