Sedentary Lifestyles aren't Necessarily Easier
The problem with being a sedentary farmer is that once you get your seeds in the ground you now have a long standing commitment to care for that plot of land. Folks don't really know nowadays because all our food comes from the grocery store, but pre-industrial farming ain't easy. Its 12 hour work days 7 days a week for months on end, followed by a frenzy of panicked activity. This burst of frenetic labor will hopefully result in a harvest, weather, pests, crop choice, soil chemistry, and water availability permitting. If just one of those variables goes wrong guess what? You better be ready to heavily reduce or quit that whole eating habit until next year. Contrary to popular belief, people did not always experience famines in the dead of winter unless that year was REALLY bad. They starved in the summer because that's the period farthest from the last harvest. Imagine the maddening feeling of starving to death while luxurious green lush plants that aren't bearing fruit yet are growing from the soil. Pre-industrial farming was not fun. It was a ton of work, sometimes for little to no return on investment.
The other problem in a situation where social cohesion has broken down is that you are stuck defending that plot of land. Those lush green crops coming in perfectly (thanks in no small part to your and your entire clan or tribe's slavish back breaking labor) are a huge billboard that says "You could probably pillage the crap out of us." So on top of the grueling labor you also need to figure out how to defend a vary large plot of land. In summary, the reasons why farming sucks are back breaking labor, predatory neighbors, and no guarantee that you will even get fed once it's all done. Last but not least, if things don't go your way you are stuck there suffering through it. You cannot just pack up an entire farm and move it to someplace where conditions are better and attackers are farther away, right?
Or could you....?
Yes You Can
Nomadic Herdsmen are in the simplest terms, mobile farmers. They are simply raising animals instead of crops. If the rain is bad and there isn't enough water for the herd, pack it up and move it. Not enough grasslands to graze? Pack it up and move it. Hostile neighbors you don't think you can fight and win against? Pack it all up and move it. Your options for handling crisis are a lot wider than that of a sedentary farmer. The attributes of this lifestyle have throughout history been attractive enough for entire cultures to form around nomadic herding, many of which have been quite successful and prosperous. When there is enough unoccupied land and there are suitable domesticated species to herd you will always find herdsmen moving from place to place with their animals.
Nomadic Herdsmen Have Historically Been Violent
There is a major downside to being nomadic, and its that you can't really specialize in anything. Those finer things in life that require complex manufacture are not going to be made by nomadic people. See, sedentary farming has its drawbacks, but once it has gotten up and running it tends to produce enough surplus for people to not always have to be farming. These people use this free time to specialize in stuff like making complex goods, selling rare and exotic goods, soldiery and weapons making and then trade their services or wealth with the farmers for food (or declare yourself leader and tax everybody for it). To be sure, tribesmen are usually perfectly willing to trade for things they can't make. But the other problem is that when sedentary agricultural societies begin to do well they tend to explode in population requiring more land, more water, and more space.
After a while the combination of shrinking pastures, lack of complex resources and wealth, and a general tough and mobile unattached lifestyle makes raiding the dirt farmers for things you want more and more attractive. They are rich, you are poor, they are fat, you are scrappy, and just look at em, sitting there with all that cool stuff so much more than they could ever possible carry. Just growing food on all that land they have and not even raising one cow! They're basically begging to be raided!
After a while nomadic herdsmen figure out that following a bunch of dumb animals everywhere isn't as cool as it used to be compared to how profitable jacking a bunch of farmer's stuff is. They still do the nomadic herd thing, they can't be raiding all the time. But come raiding season (usually late summer and early fall) its time to go get some cool new goodies, free coin, and maybe try out some different women who don't smell like horses and cow paddies. Believe it or not entire nations were (pardon the very distateful pun) sacked by nomadic raiders because they had exceptionally pretty women. It is an unfortunate fact of history that rape was a very major motivating force for raids.