Nah, the geas was established since the start of the knights, plus Hippo mentioned it a couple chapters back when facing spider.
IMO, the reason behind spider's action is to set a precedent for future knights, for both a bottom line of what is considered honorable and the importance of other knights to regulate the rogue. This was ideal opportunity to do so, since; although he was ostracized for his exceptional strength, his equally strong pupil was able to deal with it differently, by bringing knights together.
For a real world analogy, imagine if the sitting president recognized the VP as being super honorable/popular and then committed a crime, then insisted on being arrested and prosecuted, just so that future generations would have a clear system in place to deal with future bad presidential behavior.