After escaping to Milton, why do the Lytes return to Boston?

Just as Mrs. Bessie predicts, the Lytes are driven out of Milton after only a few weeks there. It turns out that Mrs. Bessie helped them flee before the mob arrived. Cilla was supposed to bring the silver with her to prevent it from being stolen, but she lost it in the confusion. She feels guilty about this and wants to go back and rescue the silver, so Johnny agrees to accompany her with a horse borrowed from Dr. Warren.