In Chapter 5 of Great Expectations, what apparently is the cause of the hostility between the two convicts?

Pip's convict wants to be absolutely sure that the second convict does not escape.  The apparent cause of the hostility between the two apparently stems back to the trial which put them both in prison.  Although the exact nature of what transpired at that trial is not described, the reader gets the sense that the second convict might have somehow betrayed the first convict.  The first convict says,

"Do you see what a villain he is?  Do you see those grovelling and wandering eyes?  That's how he looked when we were tried together.  He never looked at me".

The first convict is single-minded in his efforts to make sure the second convict is returned to custody.  Before the pursuers had even come upon the two, they could hear the sounds of scuffling, and one voice crying "Murder!" while the other voice called "Convicts!  Runaways!  Guard!  This way for the runaway convicts!"  The two criminals are pummeling each other enthusiastically when the pursuers get to them, and when they are dragged apart, the first convict declares, "Mind...I took him! I give him up to you!"  Even though the first convict knows that he too will be brought back to prison, he does not seem to care, as long as his fellow prisoner does not get away.

The second convict says that the first convict tried to murder him, but the first convict vehemently denies this charge, asserting that if he had wanted to murder him, he would have done so.  The first convict says that it would be worse for the second convict to have to return to prison than to die, and that he is determined that the other suffer the most unpleasant fate possible.  So intent is the first convict that the second convict, whom he calls a gentleman, not "profit by the means (that the first convict has) found out...(and) make a tool of (him) afresh and again", he gives up his own chance to escape to bring the second scoundrel back to justice (Chapter 5).