As she told you, Usagi Kamahime ran into him during a trip to Ryoko Owari a few years ago when they were teenagers. She "bumped" into him and inadvertently caused offense. The Scorpion, likely picking on Usagi as an extension of the already tense relations between the two clans and due to her lesser position as a member of a Minor Clan, challenged her outright. Though lacking in the full status of an adult samurai at the time the adolescents nonetheless attempted to settle their dispute in an imitation of an adult duel. That is, she ended up beating him with a stick until he ran off while trying not to cry.
Already having motive, the championship has seemingly provided him with both means and opportunity to settle the score. As a competitor Kamahime-san is highly visible, making it relatively easy for someone with an existing grudge to have noticed and located her.