View attachment 174200 Imperial Government turning Nomads is not just a Start thing.
During my Game, several times Vassals of mine turned independent and Nomad.
In addition to the situation in the picture, A rebel leader went nomad after I defeated him, and once after pressing a claim on the Kingdom of Cyprus with a female minor against a male minor, she and the original title holder both went nomad, resulting in both losing their title to adults and becoming independent.
I am not sure, but I think this situation was caused by a rebellion as well.