Some of the basic explanations are obvious, whether it is witchcraft, or some kind of unbalanced mind on her part due to her being an orphan or some other deficit which she suffers because she does not have parents to raise her.  In Act II, scene 4, Proctor's explanation is just that she is crazy, in particular that “the little crazy children are jangling the keys of the kingdom, and common vengeance writes the law!”

He later goes on, for example in Act III, scene 3 to play along with the judge and other's opinion that her weakness or madness is the fault of her un-Christian behavior and that she is deserving of a curse or some kind of bad luck due to the fact that she laughs during prayer or dances naked at night, etc.