Maybe if your tribe has a lot of cattle that needs protecting. Even than you have to do a lot of hand-waving.
Domesticated herds were the perfect article of plunder. They could easily be captured, moved, and absorbed into the plunderer’s herd. And from the other side, once people’s herds were stolen, they themselves would die off unless they in turn raided and stole from their neighbours. In this way a cyclical sequence of population density increase, plunder of cattle, and counter- plunder were set in motion in a given pastoral region.
These waves of warfare and counter-warfare expanded into tidal waves of total warfare, massive genocide, and frantic migration. And with ever desperate migration, new waves of total warfare and genocide precipitated further migrations ... and so it went.
Under such desperate condition it is no wonder that military organization was developed to its utmost extent and that the herders were to develop into the most militarized and military minded societies that the world had ever seen. And so, not only did warfare become a constant threat and a constant obsession, but, in contrast to the situation of the horticulturalists, no defense against warfare was possible for the herders. For most herders lived on open pastureland where there were no forests to hide in. And since the herders were nomadic, they could not build permanent walled settlements. Thus, possessing no walls, they erected a military structure that could ring them as effectively as the walls of the horticulturalists. They organize the entire male society into age-graded regi- ments, which would fight locked together in phalanxes as stones were locked together in walls, and would fight to the death as they were pledged to each other in life.
R.M. Glassman, The Origins of Democracy in Tribes, City-States and Nation-States,
So make your tribe having very little land suitable for agriculture, maybe just land by the river is worth tilling since you could irrigate it, say something like 10%.
60% is a steppe where it rains 3 time a year and you could only keep cattle, camels, or giant elands take your pick.
The remaining 30% are steep mountains where you could do only forest gardening but you can't do any tilling. Terraces being unknown technology to your tribe.
Then you have a organizational problem of making egalitarian tribes into hierarchical organization. Maybe the wars with neighboring tribes wasn't going so well so they put some of their mercenaries as a war chief to reorganize their warriors in manners of whatever Romans/Parthians/Chinese were doing. Taking in account local circumstances. Tribes don't have much surplus to have much of professional warriors so semi-professionals will have to do.
How he is gonna make egalitarian tribesman to do military drills and that they should listen to their "officers" is a good story all in itself. However pending doom is a good incentive for culture to change.
Plus some kind of laws for punishing undisciplined tribesman both those who are shirking and those overeager who want personal glory. Perhaps taking their good land by the river or their cattle and redistributing it to those who served with honor & discipline.
All in all plausible but unlikely.