lines 1356-1390: Infer multiple reasons why Danforth starts to believe that Mary Warren is indeed possessed by the Devil when he says, inlines 1369-1371, "why did you turn about this past two weeks? You have seen the Devil, have you not"?
I think the main reason is that Danforth wants to believe Mary. The witch court, and his job, depends on witches being found in the places they visit. Finding witches validates all his power.