What happened (if Chesterton is right, and I think he is) was that Dickens "fell in love" with Boffin, with the result that the character "got out of hand" or, in other words, asserted the freedom of its nature. This kind of thing does happen to characters from time to time--never, of course, to the puppet-character, but only to those that have received a full measure of the author's life--and their escape from control is the measure of their free will. What is particularly interesting here is the method adopted by Dickens to bring plot and character back into cooperation. He took what should have been the right way out of the difficulty, but so clumsily that the result was unconvincing and false.
The process which I shall now try to explain is something for which the reader must take my word. I cannot easily point to any successful examples in literature, because it is the whole essence of such a process that, if it is successful, nothing in the finished work will betray it. I can only state, as a matter of experience, that if the characters and the situation are rightly conceived together, as integral parts of the same unity, then there will be no need to force them to the right solution of that situation. If each is allowed to develop in conformity with its proper nature, all will arrive of their own accord at a point of unity, which will be the same unity that pre-existed in the original idea. In language to which we are accustomed in other connections, neither predestination nor free will is everything, but, if the will acts freely in accordance with its true nature, it achieves by grace and not by judgment the eternal will of its maker, though possibly by a process unlike, and longer than, that which might have been imposed upon it by force.
From The Mind of the Maker by Dorothy L Sayers, 1941, pg 78-9