All very nasty! Traditionally, when corpses were prepared for the grave, coins were placed on the closed eyelids and a bandage placed round the chin to keep the eyes and jaw closed during the process of rigor mortis - literally, the stiffening of the tissues that occurs during the first few hours after death (in humans, typically after about 2 hours, and which gradually passes off over the next 72, beginning with the head, and passing off in the same order, head first.) The stiffening causes the jaw to gape open - hence the bandage. Once the stiffness has passed, the corpse becomes floppy, and the closed jaw would 'drop', especially if - as in the case of Marley - the corpse were standing upright.
Part of the horror-factor in Dickens's story is Marley's Ghost presented as a corpse: in his final revelation to Scrooge, Marley is dressed for the grave as well as weighed down by his chains and strong-boxes, human (and dead) as well as fantastical. Dickens works a double horror, on Scrooge, and on the reader: the macabre image of the corpse releasing the bandage and its dropping jaw is the moment when Scrooge believes in the Ghost ("I do, I must...") - until now he has scoffed at its existence. And there is surely the inevitable mirroring of the Ghost's literal jaw-dropping with Scrooge's 'jaw-dropping' horror with ours - a horror worked on the reader, for we 'gape' when we witness shocking spectacles...this scene was intended to shock, and it does.