The chapter of this great book that you want to go back over is Chapter Three, which details what happens when Pip returns to the marshes and the graveyard with the food that he has stolen, as he was ordered to do by Magwitch. Let us remember that Magwitch has threatened Pip with being eaten by a "young man," and Pip is surprised to come across this "young man" as he returns, though when this person realises that Pip is there, he reacts violently:
...he swore an oath at me, amde a hit at me--it was a round, weak blow that missed me and almost knocked him down, for it made him stumble--and then he ran into the mist, stumbling twice as he went, and I lost him.
However, what is surprising is the way that Magwitch responds to being told about Pip seeing this other convict. Magwitch, when he has asked Pip about what this convict looks like, then becomes incredibly angry and demands that Pip shows him where the prisoner is:
"Where is he?" He crammed what little food was left, into the breast of his grey jacket. "Show me the way he went. I'll pull him down, like a bloodhound."
This is surprising given that Magwitch had said that he was here with the "young man." We of course realise that this was a fiction to scare Pip into complying with his wishes. However, the young man turns up in reality in the form of another prisoner that has escaped, and one with whom Magwitch has obviously had a personal disagreement, as is shown by his desire to hunt him down, "like a bloodhound."