Why does Roger shove his way past Jack, only just managing not to edge him aside?

Roger, always eager for violence, is edging past Jack in order to "show him" how to attack SamnEric.  Roger is the symbol of animal brutality, but it is a brutality barely contained by authority.  Jack is the authority.  Roger is shoving Jack, almost but not quite pushing him aside, in a symbolic action to discard human morals and turn fully to animal desire.