I think echoes in the scenes where they're leaving Brokeback and then Signal provide some clues to interpreting the "why doncha then" scene.
Both scenes involve a fight, and "Why doncha then" is kind of Ennis' verbal equivalent to his punch on that long-ago day. Both scenes involve missing being together in August. Both involve breaking camp, and the two working to get Jack's truck ready to go. Both involve Jack standing at the open door of his truck and saying he's going to go visit his folks. Both involve Ennis collapsing in tears, and snapping at someone and saying fuck ("what the fuck you lookin at" to the passerby and "get the fuck off me" to Jack). Both partings involve snow and cold: in the first one, literally, in the second one, only suggested in Jack's prediction "gonna snow tonight for sure," and his complaint about the cold. Both times Ennis appears angry but his real emotions are quite different.
Both times they part -- the first time for four years, the second time forever. In the middle of the second, we see a flashback from the era of the first, where Ennis says "see you in the morning." The first time, he does see Jack in "the morning," metaphorically -- four years later. The second time, he doesn't.
Re Ennis' breakup with Cassie: I don't get you either, Ennis del Mar. It doesn't make much sense, does it? Some people won't like this explanation, but I am coming to believe there's something messed up about the time frames of those last few scenes -- it's just too hard to believe that Ennis and Cassie were together for five years for a number of reasons.