The answer to this question can be found at the very beginning of Chapter 3. There, we see the incident in which Maniac first meets Amanda. This is on what was apparently Maniac’s first day in Two Mills after the “lost year” that he spent on his way from Hollidaysburg to Two Mills. At the beginning of the chapter, we find out that Maniac thinks that Amanda is, like him, running away from home. This is why he talks to her.
As we know by this point, Maniac has run away from “home.” He had been living with an aunt and uncle after his parents died. But he hated the fact that the aunt and uncle had a terrible relationship and he ran away. After a year, he shows up in Two Mills. There, he sees Amanda walking along carrying a suitcase. He believes that she is carrying it because she is running away. As the narrator says
What made Amanda different was that she was carrying a suitcase, and that’s what caught Maniac’s eye. He figured she was like him, running away…
Of course, Amanda was not really running away. Instead, her suitcase was full of books. So Maniac was wrong, but at least it allowed him to meet someone in the new town.