This happens when you let the inexperienced juniors try to do their own shit and not take the advice from the superiors. The older knight knew, at first sight, she was too much of a trouble for their small army to deal with, but lo and behold, our junior had to have a big mouth. If things went according to plan, the knights would come back to the Empire and tell them the existence of the third witch was confirmed and an expedition would be needed to deal with it.
I've heard good things about the novel so I might pick it up sometime soon enough.