Two additional models for nobility formation is:A tad late, but I would like to thank all of you for contributing to my topic at hand. I think one answer I didn't really think about was the slight resource differences between tribes, I think that makes a lot of sense.
1. That a nomadic tribe imposes an overlordship over a sedentary tribe.
Their heightened mobility (say, skill at horsecraft) gives them an edge in executing swift punishing raids, and also protects them against retaliative action.
2. Elite specialization among a sedentary tribe to face external crises:
A legal bargain is put up, in which the Defenders are granted a heightened judicial status for their risk behaviour.