What had prompted Orlando to accept the challenge of an accomplished opponent?

Charles then informs Oliver that he has learned that Orlando plans to challenge him the next day in the Duke's presence. Since Charles is fighting for his reputation, he indicates that he might end up hurting Orlando and he hopes that Oliver can dissuade his brother from challenging. Oliver cruelly tells Charles that Orlando has been plotting against his life, and that if Charles defeats Orlando but does not seriously injure him then Orlando will likely plot against him as well. Charles promises to hurt Orlando as much as possible, to the point where he cannot walk anymore.