Throughout the course of Great Expectations, Pip does run into some situations that seem very coincidental. These incidents serve to tie everyone together into a related web, something that Dickens likes to do in most of his stories. In a lot of his stories, there are many, many characters, but they are all tied together somehow in a cross-connected web of relationships, past ties and situations.
For example, think of the woman that is a servant in Jaggers' home. Pip eventually discovers that it is this very woman who is Estella's mother, AND the woman that Magwitch, Pip's benefactor, was in love with and married to. Estella, the beautiful and aloof orphan that Miss Havisham adopted, turns out to be the very daughter of the man that sets Pip up with his money. That is a very interesting coincidence.
Other coincidences lie around the enigmatic Miss Havisham and her disappeared lover. It turns out that he was involved in scheming Magwitch also--so, the man who jilted Miss Havisham was duped by Compeyson, the same man who duped Magwitch and led to his demise. And interestingly, those events led to Pip's expectations, his relationship with Miss Havisham and Estella, and to valuable lessons that Pip learns througout the story.
Pip's kindness to Magwitch as a boy could be seen as the determining factor for much of the story, and coincidentally, tied directly to his fortune, as Magwitch is so grateful that he gives his money to Pip.
All in all, there are many factors in the story that seem like coincidences, and serve to tie all of the characters and their lives together in personal and important ways. I hope that helps a bit; good luck!