The answer to this question comes towards the beginning of this final act, as Danforth interrogates Marshal Herrick and asks him when Reverend Hale did arrive at the jail. Note how he asks Herrick, "suspiciously", what he is doing at the jail. When Herrick responds that Hale is here to sit with those that are to be hung, Danforth responds:
That man have no authority to enter here, Marshal. Why have you let him in?
Of course, what Danforth is frightened of is that Hale and those on his side are usurping his authority and being critical of the judgements that the court has made. It is fascinating to study the character of Danforth, for he remains to set in his ways and convinced of the justice and rightness of his cause even when everything else points to a much more logical solution. He is so convinced that he is right even when innocents are hanging for his pride and arrogance, which is of course what Hale has realised and is trying to prevent, even if it means that those to be hung must lie to be saved.