Nomadic people move from area to area while settlers stay situated in one place. An example of nomadic people are gypsies. Nomadic people generally graze livestock from one pasture to another and the weather has a lot to do with where they go. They move where the weather is more favorable. When it begins to get cold in any given region they will simply move to another, more suitable region. Settled people on the other hand stay in one area. They may graze livestock and grow crops but they do not leave.
This is where the conflict begins. Nomads may intrude on the settlers space and this causes trouble. Their livestock eats from pastures that belongs to the settlers and so forth.
Typically, this is for what you might call economic reasons. The needs of nomadic people and those of settled, agricultural people tend to be in conflict with one another.
The major thing that nomadic people need is space. They need to be able to move from place to place at any given time. This means that they must have access to the places they need to be and the places in between (so they can travel from place to place).
By contrast, settled people need to have their own property, so to speak. They need fields to grow their food and to graze their animals. When they get these, they need to keep others off of these areas.
This is where the conflict arises -- nomads need lots of space and free access to that space. Settled people try to take that space and cut off access to it. This leads to conflict.
Example: cattlemen and settlers in the Old West. The cattlemen wanted open range for their cattle to graze and to be able to drive the cattle across to market. The settlers needed fenced fields that would be unavailable to the cattle.