Factual error: The geographical context of the film is wrong. Riverton, Wyoming (as the name implies) is in a fertile agricultural river valley. The surrounding country is uncultivated desert. There is none of the extensive grasslands shown in the film. Moreover, even in the 1960s Riverton, which is a county seat and market center, was much more substantial than the film implies.
Factual error: The deer shot by Ennis is not an elk but a European red deer. There are no wild red deer in the USA, only imported and fenced in.
Answer: At first they were just wrestling and horsing around. However, underlying tensions and frustrations over their impossible situation, and Ennis' reluctance to commit to Jack, causes their anger (particularly Jack's) to overtake them, and it erupts into a full-blown fight.
raywest ★