I would appreciate learning more about how pretenders to a khaganate can split off to form new realms of their own upon the ascension of the new khagan, taking territory with them. Under what conditions can this occur, and can anything other than abandoning nomadic government be done to prevent this from occurring?
Nomad clans might break away after a succession under two conditions: 1) if the new ruler is a different dynasty than the last, or 2) if the nomadic realm is large (25+ provinces and at least 4+ provinces in the ruler's personal demesne) and the previous ruler had more than one capable adult son.
In the case of #1, it really depends on how much a clan leader likes the new ruler. In the case of #2, it will automatically split up the demesne/population as if the ruler had gavelkind succession and the brothers will decide if they wish to secede from the realm -- if they do, any clans that neighbor their lands will be asked if they want to join them.