Why does Magwitch jump with sudden fear when Pip points to where his mother is?

Abel Magwitch is an escaped convict, on the run after breaking free from a prison ship. The soldiers are hot on his trail, combing every inch of the Romney marshes to find him. As we can well imagine, he's absolutely scared stiff at the prospect of being caught. Being locked up in a prison barge must have been a pretty wretched experience, so it's not surprising that he really doesn't want to go back there. When Magwitch first encounters Pip the boy is in the cemetery visiting the grave of his parents. As Magwitch is so jumpy at the prospect of being caught, he immediately thinks that Pip's mother is actually there with him in the cemetery, physically present, rather than dead and buried beneath the ground.