Cattlemen and
Homesteaders conflict.
One of the reasons that cowboys and cattlemen came into conflict was due to the ticks. The ticks were a type of flea which lived on the Texas Longhorn cattle on the drives onto the plains. Although they were immune to the fleas, the homesteader's cows and other animals were not, and so this killed many of their animals, leading to disputes and trying to ban the cowboys from driving cattle across their land - this conflict was one of the reasons for the end of the cattle drive.
Reason 3 - Sheep Farmers
Sheep farmers were a severe threat to the cattle ranchers as they competed for grazing land - in some states the number of sheep outnumbered the cattle. Sheep rearing allowed for a smaller initial investment and a quicker return - therefore many people wanted to become sheep farmers.
However the cattle farmers responded to this with violence - killing sheep and shepherds and burning sheep hay. As many of the shepherds were immigrants, racial and religious discrimination may have been a contributing factor in this violence.
Reason 2 - Barbed Wire
Barbed wire initially meant that homesteaders were able to mark off their boundaries and protect their crops from stampedes and disputes over land and such things.
However when the cattle ranchers began to move onto the Plains this caused more disputes than it solved. When the homesteaders used it to fence of their land (to protect it from straying cattle and wild beasts), this initially brought about the hostility of the cattle ranchers as often they fenced off large areas of land stopping the ranchers being 'open range' - this resulted in many cattle being cut off from water supplies.
When the larger cattle barons began using barbed wire to also fence off large areas of land for them, smaller ranches faced the danger of being cut off entirely - meaning they had to resort to fence cutting, a crime.
Why this was.
Reason 1 - Ticks