User blog comment:TheBlueRogue/The Witcher 3 "The Trail" Fannotation Trailer/@comment-31.182.83.8-20141029171132/@comment-78.96.209.40-20141029171910

Since the horse head is still rather fresh when Geralt checks it, I think we can assume they were close on her trail. Since according to the Wiki Timeline this was the year the witch hunts started, she probably wandered unto the battlefield and the officer, recognizing or assuming she's a witch on the run tries to get a bonus kill for fame. Since they're looking for her, Geralt hasn't lost his memory yet (W1 starts the following year). The movie is probably a flashback in itself.