I think its a bug that casues the Origin to switch whenever you use the Unite feature. In my game a split off nation (Holland, origin Frisia)  was then United by the realm it split off from(Frisia, origin Frisia), and then when I looked at the origin map mode the part that used to be Holland completely switched origins to Britano-Roman. I dont know if theres some features exsclusive to the Origins map mode that are messing with it, but it makes it a pain to try and fix.