I can't remember exactly what is said, but after Captain Jaggery orders Mr. Hollybrass to lash Zachariah with the whip fifty times, he questions him and then hesitates to do what he was told to do.
Here Scrooge discerns another reason for the presence: his old friendship with Marley. In this moment, Marley's desire to help out his old friend has caused the ghost to appear.
