I think it is surprising but also a really remarkable way of connecting all of the stories together. Upon learning who Pip's mysterious benefactor is, we can begin to put all the pieces of Pip's story together. It would be logical for Miss Havisham and Magwitch to have the same lawyer because of Compeyson. Compeyson was Magwitch's partner and he also is the one who jilted Miss Havisham. When Magwitch needed a lawyer, it makes sense he would turn to Jaggers because he probably knew of Jaggers from Compeyson and/or Miss Havisham. And with Magwitch in jail, it also makes sense that Molly, Magwitch's wife had no means of support and turned to crime. Jaggers arranges for her daughter to be brought up by another client who wanted a child, Miss Havisham. Jaggers had no idea at the time that Magwitch would escape one day and find Pip, who lived also lived near Miss Havisham and wanted a playmate for Estella. I think Dickens showed a brilliant way of connecting all the dots of his story into one cohesive plot.