As the rut nears, bull elks start acting a bit strange. As their internal body temperatures rise, male elk will begin seeking out pools of mud to cool off and add an intimidating layer of dirt. The older, more confident bulls will start to gather harems and fend off younger males trying to butt in. Younger males rarely have a chance, however, and generally spend them the time sparing with other bulls their own age.
During this time the animals also shed the last of the velvet covering their antlers, either by rubbing them on trees, rocks, or even the ground itself. For the observer, these displays can sometimes appear to be confrontational, as if the bull wanted to pick a fight with the grass.
Of course, with the sudden surge of aggression that many elk go through during this period, maybe they really wanted to fight that grass. It also happens to be good practice for when a real fight occurs.